Spanners, only just caught up with this thread. Did you say you were getting screen corruptions before freezes? Like some pixels just aside of the the mouse pointer?
If so, my guess is a memory corruption problem, either caused by bad drivers for the video card, or bad configuration between the video card and bios, or just a plain broken video card doing duff memory cycles.
If the problems go away with another brand of card installed, you know the answer

Old PCI cards work fine, just received replacement card, which I shall install very soon. Hopefully all will be okay then, fingers crossed. I thought it was drivers but used Creative and Nvidia with the same results. BIOS has been updated and all settings correct according to both manufacturers. Voltage set at 3.5v. Here's hoping!!!!!

Just installed replacement card, so far so good no fre.....

Clean all SDRAM, PCI and ISA contact edges, ensure they are sitted home.