uiSlider thumb graphic 'flashes' when dragging uiScrollview on 1st gen 3.1.3

I have added my own thumb to a uiSlider and added a 1px x 1px transparent .png to the tracks (I don't want to see the tracks) (I have also tried setting tracks to nil).

My uiSlider is also rotated to vertical.

The uiSlider sits over a uiScrollView but is not 'attached' to that view. There are numerous uiLabels 'attached' to this uiScrollView, so they all slide about 'beneath' the uiSlider.

It all works fine on 3g and iphone 4, but on my 1g itouch (3.1.3) I get a strange flash from the thumb graphic sometimes when scrolling the uiScrollview!?

It does seem to happen when the edge of a uiLabel (on the uiScrollView) passes 'under' the thumb graphic?

I have tried everything I can think of...

Got it! made the 1px x 1px transparent .png 2px x 2px instead!