Xcode Keeps Crashing. What should I do?

Reinstalling, uninstalling, reinstalling 4.0.2 doesn't work

Process:         Xcode [649]
Path:            /Developer/Applications/Xcode.app/Contents/MacOS/Xcode
Identifier:      com.apple.dt.Xcode
Version:         4.0.2 (99)
Build Info:      IDEApplication-990000~53
Code Type:       X86-64 (Native)
Parent Process:  launchd [91]

Date/Time:       2011-05-15 22:13:29.007 +0700
OS Version:      Mac OS X 10.6.7 (10J869)
Report Version:  6

Interval Since Last Report:          128065 sec
Crashes Since Last Report:           7
Per-App Interval Since Last Report:  118616 sec
Per-App Crashes Since Last Report:   7
Anonymous UUID:                      B0FCFF42-38B8-4466-8161-8D3F5CEBC494

Exception Type:  EXC_CRASH (SIGABRT)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Crashed Thread:  0  Dispatch queue: com.apple.main-thread

Application Specific Information:
ProductBuildVersion: 4A2002a
Encountered multiple assertions. First assertion was: UNCAUGHT EXCEPTION (NSRangeException): *** -[NSCFString _getBlockStart:end:contentsEnd:forRange:stopAtLineSeparators:]: Range or index out of bounds
UserInfo: (null)
Hints: None
Backtrace:
  0  0x00007fff80b40796 __exceptionPreprocess (in CoreFoundation)
  1  0x00007fff821e50f3 objc_exception_throw (in libobjc.A.dylib)
  2  0x00007fff80b405d7 +[NSException raise:format:arguments:] (in CoreFoundation)
  3  0x00007fff80b40564 +[NSException raise:format:] (in CoreFoundation)
  4  0x00007fff80cb64aa -[NSString _getBlockStart:end:contentsEnd:forRange:stopAtLineSeparators:] (in Foundation)
  5  0x00007fff823f202d _NSFastFillAllLayoutHolesForGlyphRange (in AppKit)
  6  0x00007fff82948cbb -[NSTextView(NSPrivate) _ensureLayoutCompleteForVisibleRectWithExtensionFactor:minimumExtensionDistance:repetitions:] (in AppKit)
  7  0x000000010020d1a3 -[DVTSourceTextView(FoldingSupport) _ensureLayoutCompleteForVisibleRectWithExtension:] (in DVTKit)
  8  0x00007fff823ef694 -[NSTextView setNeedsDisplayInRect:avoidAdditionalLayout:] (in AppKit)
  9  0x0000000100204b15 -[DVTSourceTextView setNeedsDisplayInRect:avoidAdditionalLayout:] (in DVTKit)
 10  0x00007fff823acc3b -[NSView setNeedsDisplay:] (in AppKit)
 11  0x0000000100075614 -[NSObject(DVTPropertyLinking) dvt_addObserverWithName:toKeyPath:ofObject:block:] (in DVTFoundation)
 12  0x0000000118f0b833 -[IDESourceCodeEditor contentViewDidCompleteLayout] (in IDESourceEditor)
 13  0x0000000118f0b115 -[IDESourceCodeEditorContainerView didCompleteLayout] (in IDESourceEditor)
 14  0x00000001001d8fc0 +[DVTAutoLayoutView _recursivelyLayoutSubviewsOfView:populatingSetWithLaidOutViews:] (in DVTKit)
 15  0x000000010088c2c9 __66-[IDEEditorContext _openNavigableItem:documentExtension:document:]_block_invoke_0 (in IDEKit)
 16  0x000000010088be38 -[IDEEditorContext _performBlockInsideReentrantGuard:] (in IDEKit)
 17  0x000000010088bd08 -[IDEEditorContext _openNavigableItem:documentExtension:document:] (in IDEKit)
 18  0x00000001008bfd31 -[IDEEditorContext _openNavigableItem:documentExtension:] (in IDEKit)
 19  0x000000010088b50f -[IDEEditorContext _openNavigableItem:] (in IDEKit)
 20  0x00000001008bf033 -[IDEEditorContext _notifyDelegateAndOpenNavigableItem:documentExtensionIdentifier:locationToSelect:annotationRepresentedObject:stateDictionary:annotationWantsIndicatorAnimation:exploreAnnotationRepresentedObject:] (in IDEKit)
 21  0x00000001008bb678 -[IDEEditorContext _notifyD开发者_如何学编程elegateAndOpenEditorOpenSpecifier:updateHistory:] (in IDEKit)
 22  0x00000001008bb453 -[IDEEditorContext _openEditorOpenSpecifier:updateHistory:] (in IDEKit)
 23  0x00000001008bb15e -[IDEEditorContext openEditorOpenSpecifier:] (in IDEKit)
 24  0x00000001008bb13b -[IDEEditorModeViewController openEditorOpenSpecifier:editorContext:] (in IDEKit)
 25  0x00000001008bb0f9 -[IDEEditorArea _openEditorOpenSpecifier:editorContext:takeFocus:] (in IDEKit)
 26  0x0000000100941bcb __108+[IDEEditorCoordinator _doOpenEditorOpenSpecifier:forWorkspaceTabController:editorContext:target:takeFocus:]_block_invoke_0 (in IDEKit)
 27  0x00000001008bae5d +[IDEEditorCoordinator _doOpenWithWorkspaceTabController:editorContext:target:allowFallback:documentURL:usingBlock:] (in IDEKit)
 28  0x0000000100941ae8 +[IDEEditorCoordinator _doOpenEditorOpenSpecifier:forWorkspaceTabController:editorContext:target:takeFocus:] (in IDEKit)
 29  0x00000001008ba899 -[_IDEOpenRequest _runIfNecessary] (in IDEKit)
 30  0x00000001008ba2a1 -[_IDEOpenRequest _enqueueForEventBehavior:] (in IDEKit)
 31  0x00000001008b9dab +[IDEEditorCoordinator _openRequestForEditorOpenSpecifier:workspaceTabController:editorContext:eventBehavior:takeFocus:] (in IDEKit)
 32  0x00000001008b9d00 __99+[IDEEditorCoordinator _openEditorOpenSpecifier:forWorkspaceTabController:eventBehavior:takeFocus:]_block_invoke_0 (in IDEKit)
 33  0x00000001008b9c90 _performBlockInsideReentrantGuard (in IDEKit)
 34  0x00000001008b9bff +[IDEEditorCoordinator _openEditorOpenSpecifier:forWorkspaceTabController:eventBehavior:takeFocus:] (in IDEKit)
 35  0x00000001008b98e9 +[IDEEditorCoordinator openEditorOpenSpecifier:forWorkspaceTabController:eventType:] (in IDEKit)
 36  0x000000011d964ed0 -[IDEStructureNavigator addTemplatesFromCompletedContext:] (in IDEStructureNavigator)
 37  0x00007fff80ca0562 -[NSBlockOperation main] (in Foundation)
 38  0x00007fff80c90cc4 -[__NSOperationInternal start] (in Foundation)
 39  0x00007fff80d6fae7 __doStart2 (in Foundation)
 40  0x00007fff80d6fcdf ____NSOQSchedule_block_invoke_1 (in Foundation)
 41  0x00007fff865f2284 _dispatch_call_block_and_release (in libSystem.B.dylib)
 42  0x00007fff865d0df2 _dispatch_queue_drain (in libSystem.B.dylib)
 43  0x00007fff865d169f _dispatch_queue_serial_drain_till_empty (in libSystem.B.dylib)
 44  0x00007fff8660443c _dispatch_main_queue_callback_4CF (in libSystem.B.dylib)
 45  0x00007fff80adc0c8 __CFRunLoopRun (in CoreFoundation)
 46  0x00007fff80adadbf CFRunLoopRunSpecific (in CoreFoundation)
 47  0x00007fff833ae7ee RunCurrentEventLoopInMode (in HIToolbox)
 48  0x00007fff833ae551 ReceiveNextEventCommon (in HIToolbox)
 49  0x00007fff833ae4ac BlockUntilNextEventMatchingListInMode (in HIToolbox)
 50  0x00007fff823d2e64 _DPSNextEvent (in AppKit)
 51  0x00007fff823d27a9 -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] (in AppKit)
 52  0x00007fff8239848b -[NSApplication run] (in AppKit)
 53  0x00007fff823911a8 NSApplicationMain (in AppKit)
 54  0x0000000100000eec

after uninstalling xcode, use your Snow Leopard disk to reinstall the system then run all updates from scratch before installing it again. This type of reinstall shouldn't affect your data or setup.