All your base? :hmm:
Anyways, I had this issue once with a 6600gt. I replaced the PSU and I'm pretty sure I still got it a few times after that.
I think that it's most likely caused by general degredation of the silicon components. It could be a transistor is leaking a little more than usual, dropping the voltage to the cards "checkpoint" enough to set off the alarm.
Who knows? It's good that the BIOS flash took care of it for you. There's nothing more frustrating than having the hardware and not being able to use it!