whiteout Problem on IE7

Hy Folks,

I'm working on a specific bug on the site I'm developing right now for
something like a week and its driving me crazy. The Site is rendered
on IE6 and FF like intended (some minor positioning glitches), but on
IE7 a whole part of the Site just disappears after you start using it
until you force a redraw of the window (by resizing the browser i.E.).
If anybody has an idea how to get rid of that Problem I would be
infinitely thankful.

Summary:
URL: <http://ca.communalaudit.at>
User: demosubscriber
Pass: demouser

Scenario:
1. After Log in click on one of the entries in the left sidebar (i.E.
"Wasserversorgung")
2. Now start scrolling around a bit.
3. Enter some values in the input Fields and tab to the next (auto
save should occur)
4. Hover over a question text until you see an Information icon. On
Click this Icon should open a Description Field for the Question.
(this contains some invalid legacy markup which will be cleaned out
and has nothing to with the problem, i tried it)

5. Sooner or later the #main div (containing the questions) will
whiteout until you resize the browser window.

Any Help greatly appreciated

--
Yours sincerely
Milan Zoufal