|First, did you use a good thermal compound when installing heatsink? (heat build up as possible issue)|
Second, run Memtest86 to test for bad memory. (you may have a bad memory stick and only a serious memory test will find problem)
If it was during video playback but not on audio only, it might have been video card related, but if same on audio, probably not.
Not sure I understand windows 7 reference, is this a dual boot? If you are also running windows 7 on same hardware without problem, it does not necessarily eliminate the hardware possibilities since windows 7 uses resources differently. Windows 7 might be able to deal better with the cpu temperature, or some bad memory segments, at least until it gets worse.
If you are not completely positive about heatsink, reset properly. Run Memtest (download I believe from Memtest.org or memtest86.org.... there is one to make a bootable floppy, and one with an image to burn a bootable CD) Insert and reboot to test.