I have had a Gainward Ultra/760 XP (FX5600 ultra) for 3 weeks and have
been unable to run 3D games for more than about 15 minutes without the
PC freezing and having to be turned off and on. I upgraded from a
Geforce 2 which ran everything without problems. The games I play are
Everquest and UT2003 and the problem also arises in 3Dmark03. The
freeze almost always happens when a new level/zone is loading or has
just loaded and very rarely in the middle of a level.

I have:

Athlon 2500
MSI KT4VL motherboard
512 Mb Corsair PC2700 RAM
SB Live
US Robotics network card
Thermaltake 420W PSU
Windows 98SE

These are some of the many things I have tried:

Upgrading to 44.03 drivers
Upgrading to 448 4 in 1 drivers
Upgrading to DirectX 9.0b
Ensuring that video, network and sound cards are not sharing IRQs
Changed AGP aperture from 128 to 64 and 32
Disabled/enabled fast writes
Lowered hardware acceleration
Increased AGP voltage
Underclocked/overclocked video card
Removed sound and network cards

I assume my PSU is good enough and I don't think it can be
overheating.
I also tried the card in another lower-spec PC and had exactly the
same problems.
My final option is to return the card - but I don't know whether it is
a problem with this particular card or if there is some compatability
with my system.

Has anyone got any ideas please?

Thanks in advance