Has anybody found a solution to this problem yet?
I'm experiencing the exact same thing. I initially purchased the card new on eBay. After discovering the flickering problem (and underclocking, as you did), it seemed somewhat better... But, I wasn't happy with it, so I had the seller exchange it for another. The replacement did the same thing, so I figured it was a mainboard incompatability and decided to put up with it.
In November, I started having more trouble with it. SimCity 4: Rush Hour were suddenly displaying random pixels (artifacts) over the screen...just out of the blue...never happened before that. Games and programs seemed to be running chunky, too. I updated drivers (new Detonator, DirectX 9b, AGP117), but the problem persisted. So, I RMA'd it to Leadtek. While they were sending me a replacement, I had my old GeForce256 card in...no problems whatsoever (no flickering, either!). However, after installing the new card I received from Leadtek today, the problems started all over again...including the random pixels. It's driving me nuts. It's obviously not a motherboard incompatability, because everybody I've read about who's having trouble with it is using a different mainboard. I've seen posts at several other sites with people who are in the same predicament.
Here are my specs:
P4 2.53 GHz
Shuttle AS45GTR Motherboard
512 MB DDR333 RAM
Leadted Winfast A280LeTDH
Happy Holidays, BTW!