Marrs, that's exactly what I ended up doing, just playing around with the engine clocks to see where the sweet spot was at.

I had thought I found the right settings at about 975 engine and 1600 memory clocks, but after about 20 minutes or so the screen would go to a solid color, which I take it to mean the card had 'crashed'.  I figured the memory clock was just set too high and dropped it back down to 1375, of course my hash rate dropped quite a bit, so I started slowly raising it again, eventually coming up to 1425Mhz which is apparently where the best spot for my card is at.  I let that run all night and no more lockups and the hash rate is right at the 650khash/s range with nothing else going on with the system.  If I open a browser (in typing this) I saw the hash rate drop to 622khash/s, so I bumped the memory a little to 1440Mhz and the speed came back up into the 640khash/s range, which I'm happy with as long as I don't have any more lockups.

Thanks for the explanation crazyates on the internal latencies, that makes sense, and saves me from searching.