Windows 7: Black or grey&white lines crash

I hope you can help me solve this problem:
About a month ago I've upgraded my rig, and since then I'm encountering black or grey and white screeb crash when I'm watching videos on youtube or embedded youtube videos (something like this or this ).
But I can play DX9 or DX10 games like Half Life 2, Mass Effect 1 and 2 for 4-5 hours without any problems. The crash only occurs with youtube videos.
My specs are:
Windows 7 Ultimate x64 build 7600
Asus M4A88T-V EVO with integrated Ati HD4250 enabled - I don't have a standalone GPU yet.
AMD PhenomII X4 955BE, revC02
2x2GB Kyngston Hyper-X Blu DDR3 1600Mhz running at 1333Mhz with 1.5V
Sirtec HPC-500-H12S 500W PSU
WD 3200AAKS -320gb HDD
Hitachi 200GB HDD
WD 5000AAVS external drive
all in a low-end Raidmax Sagitta case with default fans.
In the attachment there are 2 archives with Minidumps because the "BSOD Dump & System File Collection app" archived only the last three minidump files, not all 7 of them.
If you need any other details feel free to ask. Thanks in advance.

This is a sign of overheating or a defective GPU
Update your video drivers ATI Radeon