Black lines/blocks issue in Screen Sharing window

I'm experiencing black lines and blocks in the Screen Sharing window when I share the screen from my Mac Mini (10.7.2) to my MacbookPro (10.7.2). After first connecting the screen looks like this or something similar.

When I move a window around on the remote computer (Mac Mini) from my Screen Sharing app the black lines/patterns go away. Like the screen has to refresh. I can 'whipe' the screen from black artifacts by dragging a remote window like a whiper around the remote screen. Sometimes they show up again later in the same session. And give results like this:

Hopefully someone has an idea how to solve this. The Mini Mac (2,33 Ghz i5, 2Gb RAM, 500Gb) is brand new, bought just two days ago. I reinstalled Lion to see if it solved the problem, but it didn't.

Thanks for any help.

Jan Peeters