Strange system font / graphics problem

Hello,
I have the problem shown in the screenshot for some days now.
I did not install anything. I tried to delete font caches but it did not help.
I re-installed the system but that did not change anything. After resetting the PRAM the problem was solved. At least for some hours, but now the problem appeared again.
When I do a reboot, strange white lines appear over the grey background and the Apple Logo, then the screen turns blue, the cursor consists of white lines too and nothing more happens. After several tries to reboot the Mac is running again, but the font/graphic problem still exists.
First I thought of a graphic card problem, but it seems that this font problem only concerns system prefernces and system applications.
Apple Hardware Test says that there is no problem.
I called Apple support but they also could not really help me. They said I shoult take the Mac Pro to an Apple Service Provider but there is none near where I live. Maybe anyone here has seen something like this before and has an idea what the problem is.
I have the New Mac Pro 2.66 GhZ, 6 GB RAM and ATI Radeon 4870.

 

So, I sent my Mac Pro back to get it repaired. Got it again today. The graphic card caused the problems and I got a new one.
Thanks to all who tried to help.