Graphics display problem

[David Keens]

After my PC's been running for about 15 minutes the display suddenly goes
like the attached screenshot - all covered with moving multicoloured blocks.

I've changed the graphics card (from a PCI card to an AGP card) with no
improvement.
I've installed new memory with no improvement

It's not the monitor since I can capture it with a screen grab :-)

If I send they system into "stand by" and then wake it up the display is
fine for a while.

The only thing I can think of is that the motherboard is starting to go...

Any suggestions???

Many thanks

David