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The PSU has two 22A rails at 12V. If the card alone is drawing 38A, that means it's pulling 456W (12V * 38A = 456W). Now the maximum that a card with two 6-pin PCIe power connectors can draw is 225W. 75W comes through the motherboard slot and the 6-pin molex connectors are each rated for 75W. (Offhand knowledge verified here) Furthermore, the raw output of a power supply is best measured in Watts. The two +12V rails in my unit each theoretically provide 264W. I don't have the right meter to verify what the rail is actually putting out, that's not easy to do from an electrical standpoint. Anyway, when a load (gfx card, cpu, whatever) starts to pull more Amps than the supply can provide, the voltage drops suddenly. Remember that Volts * Amps = Watts. The supply only has a certain number of Watts to give so to be able to increase Amps beyond it's rating, it has to decrease Volts. I put a voltmeter on the +12V connectors going right into the card and watched the voltage while the card was operating. Even under extreme load the voltage never dropped below 11.4V, which shouldn't come close to being problematic. Are you sure that's 98A? 98A * 12V = 1176W, which means that one rail exceeds the entire rated output of the supply...

I'm not overclocking, though while running a GPU monitor I have noticed that the clocks immediately switch back to the 2D when a non-fatal crash occurs. I figured that was because the 3D pipeline had terminated and it reverted to just rendering the desktop.

EDIT:
Also, isn't 850 MHz a bit of a stretch from a baseline 405 MHz? I'm not an overclocking expert (I'm sure you'll get better replies), but that might be why you've got an unstable setup.