Imac G5 - screen freezes with patterned screen

Yesterday my Imac G5 suddenly had the whole screen covered with patterns which in turn froze
the machine up.
Tried using the hardware test (both quick & extended) found nothing wrong.
Tried resetting the Pram & Nvram - made no difference
Tried resetting the SMU - made no difference
Ran Disk Utility to repair permissions - all OK
Used DiskWarrior and no problems found.

Powered off and left it all night and restarted this morning all systems appeared OK
This evening this problem returned and had to power off again which is where I am now.

I have looked thru these forums and found reference to faulty capacitors about bulging, leaking etc
and removed the back but could not see any faults

Any one have any clues as to what the problem is?

I would appreciate some help here please

Alan