To make the link more prominent, I am posting it in an answer too. It looks like a bug is within the framework code and there are no workarounds found yet. I have reported the bug on Microsoft Connect:
Microsoft Connect: Scrolling in virtualized WPF TreeView is very unstable
There is also a maybe related bug which was posted in the comments by @sixlettervariables:
Microsoft Connect: WPF application freezes while scrolling the TreeView under specific conditions
If you can reproduce the bugs, please vote them up.