Disconnect everything, DVD Drive, HDD and anything else. Just have one stick of RAM in and boot up to BIOS. (if you go past the BIOS POST it will tell you 'no bootable drive found' but ignore that) Leave pc on and see if still freezes after about half an hour.
(to switch off press in the power button on tower for about 10 secs)
Add a component one by one and test each time for freezing.
Have you disturbed the CPU at all? I noticed in another thread you were thinking of changing it.