Random Momentary Vista Freezes / Hangs

At random intervals (2 to 30 minutes apart) Windows Vista Ultimate 64 freezes
/ hangs / locks up for about 20 seconds then resumes working as normal.
During that time the Reliability and Performance Monitor show the processor
activity spikes to about 50% and then just goes blank (not down to 0%, just
blank) leaving a gap in the red line chart.

This was a fresh clean install of Windows Vista Ultimate 64. I updated all
drivers, swapped out video cards (NVidia GEForce 7600GT OC & ATI X1300 Pro),
and removed the TV Tuner (which no longer works due to Vista conflicts
anyway).

Thanks for your help.
classicmovies101

experiencing similar problems,

1st time I was running tweakvi, installed powerdvd, a few codecs, and the
problems highlighted materialised.

reformatted, reinstalled, working fine again, power dvd and codecs all
working fine. then 2 days ago, it started doing the same thing, I cannot
think of any programs that I have installed lately that might cause it,
although I did install a new driver for my netgear wireless adaptor.

This is a laptop