Video card troubleshooting

Question

In the last couple of days, I have begun having serious but intermittent (I know, I know) video artifacts on my early-2009 Mac Pro. I am running 3 displays on 2
NVIDIA GeForce GT 120 video adaptors. Basically what's happening is one of 2 phenomena: in 1, the image "shatters", or goes to pieces, with small blocks  of whatever was on the screen being scattered around the screen. Each block is maybe 3/8" wide and 5/8" tall. In the second case, moving a window around the screen leaves a trail of the border portion of the window, as though the card were having trouble keeping up... except that it doesn't then "catch up" as sometimes used to happen with older technology.
Is this a simple "fail" for which I should seek warranty replacement, or is there perhaps something that can be done to troubleshoot or further investigate first?
As background, I have not yet upgraded to Lion, so I am running OS-X 10.6.8, with weekly or better updates and so forth.
Thanks in advance for any help.

Answer 1

Right. I did this while I was awaiting replies. As with all things intermittant, it seems to have resolved once I re-arranged the monitor-card assignment. And then vanished for the last while.
Interestingly development, when I swapped the monitor cables. The rear fan came on quite loudly and stayed on, as it sometimes does with Mac Pros. I followed the ususal routine to fix the situation by putting the machine to sleep and waking it. After the first "fix" the fan started again after a few minutes. I repeated the fix overnight, and now both the noisy fan and the video glitches seem to have vanished like dew on the morning grass once the sun is on them. I hope this continues, though I have my doubts. A few re-starts earlier did not resolve the video glitch.
Thanks.