Problems with XFX FX5900XT

About 1 1/2 months ago I upgraded my Ti4600 to the above card and during
games (Doom III and Painkiller) I'd intermittently get green dots
appearing on screen, sometime it'd sort itself out, other times the
system would freeze. Not long after that I'd even get problems when on
the windows desktop vertical lines of dots and it started to cause the
monitor to try to resynch.

Tried various drivers and disabling fast writes but had same problems.

Even though my PC came with a 300W PSU i thought that it probably wasn't
a very good one and as it was powering a cd-rw in addition to a dvd-rom
and an Audigy Platinum drive bay panel it might not be up to the task.

Just installed a 430W PSU by Antec and although the computer started off
fine at the desktop, the same problems occured in the games and when I
rebooted the PC later - it rebooted a few times before windows xp loaded.

Does it sound like the card is duff or is there something else I can try?
(Have already tried re-seating the card, scanning for viruses & spyware)

thanks

Mal


System:
XP2100+
512Mb PC2700 RAM
XFX FX5900XT
Audigy Platinum
Antec 430W PSU