Apache HTTP 500 but When Refresh Render Properly

I have a website (mobile web) running in 2 server and 1 balancer with haproxy, OS ubuntu 10.10

Apache Configurations:

Timeout 300
KeepAlive On
MaxKeepAliveRequests 100
KeepAliveTimeout 15

StartServers          5
MinSpareServers       5
MaxSpareServers      10
MaxClients          150
MaxRequestsPerChild   0

The site is running well but suddenly now often it return blank page HTTP 500 (20 bytes in 30-50ms) with no error messages, but when i refresh it rendered properly.

does anyone have a clue on where it possibly goes wrong?

Update:

I dont know if this relevance but anytime it could not be loaded browsers push me to download the content, and when i checked the downloaded file (name: Random_Chars.~.part) i found that in the header of the HTML these codes added by browser

meta http-equiv="Content-Type" content="text/html; charset=utf-8" 
meta name="viewport" content="width = device-width, maximum-scale = 1.0, initial-scale = 1.0"
meta name="apple-mobile-web-app-capable" content="yes"
--- ADDED BY BROWSER --
link href="styles/reset.css" rel="stylesheet" type="text/css"
link href="styles/base.css" rel="stylesheet" type="text/css"
script type="text/javascript" src="scripts/templates.js"
script type="text/javascript"
function init() {
var mySlideshow = new Slideshow ("slideshow", 0, true);
var myToggle = new ToggleSwitch(["toggler1","toggler2","toggler3"]);
var myStyleTweaks = new StyleTweaker();
myStyleTweaks.add("Series60/5.0", "styles/tweaks开发者_JS百科/S605th.css");
myStyleTweaks.add("Series60/3.2", "styles/tweaks/S603rdFP2.css");
myStyleTweaks.add("AppleWebKit/420+", "styles/tweaks/S406th.css");
myStyleTweaks.add("N900", "styles/tweaks/maemo.css");
myStyleTweaks.add("Firefox/3.0a1 Tablet browser 0.3.7", "styles/tweaks/maemo.css");
myStyleTweaks.add("Opera Mini/4", "styles/tweaks/operamini.css");
myStyleTweaks.tweak();
}
addEvent("onload",init);
/script

In Apache Error Logs i found this line:

[client ip] File does not exist: /var/www/http:, referer: http://domain.com/search

note file filename "http:"

its sometimes not response 500 but 200 OK but the content is blank until i refresh few times Any help will be appreciated.

Check the error log of your server, it may contain helpful error messages. Don't forget to enable logging (PHP's log_errors=1; phpSettings.log_errors=1 in ZF config).

The script seemingly crashes before having a chance to set the Content-Type header, so the browser don't known what to do with the response, and this is why you see a download dialog.

Sorry I found that was APC Module caused that, i disable APC then those strange errors gone.