something very similar to what you describe has happened to me twice; last time was a few days ago;
my card was replaced by a new one first time it happened and i expect this second one to be replaced too.
I sold that 260 to Dave, and I assembled/tested the waterblock prior to shipping it out. I'd be really surprised if the thermal paste that I add smeared off from the memory chips - I've done that many, many times and never had a problem.
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