One thing that I always disable in the BIOS is the power management. My brother's PC was doing the random freeze for 2-5 secs thing until I disabled all the power management in the BIOS.
Give that a shot,
Also, try the ram in a different slot (sometimes works)
Make sure the processor is getting plenty of fresh air (clean up the cables etc) clean the dust and crap out of the heatsink.
Check that all the boards are seating properly in the MB. Soemtimes when you tighten the retaining screw onto the case, it levers the end of the board up a fraction.
Oz.
oh if none of that works, make sure your PC is aligned directly with the case facing Microsoft headquarters.