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My first guess based on your description of the failure mode and steps to recover, would be a bad power supply. I know you did a voltage test, but if it is something only occurring under specific loads of the video card, and dragging another voltage output too low, it would be hard to catch unless you have a way to log all the voltages right up to the moment of the failure. Do you have another power supply you can swap in, just to eliminate that? Or if you have multiple 12v rails, have you tried rebalancing loads (particularly as regards which 12v rail the 550ti is on)?
Also in regards to the 550ti, and your mention of "garbled sound"... The HD Audio modules on that board sometimes cause conflicts/problems. If you are using the motherboard or a pci card audio, and aren't using the Nvidia HD Audio modules on the 550ti, try disabling them in device manager -- and see if that eliminates the problem.
Also, have you tried a newer Nvidia driver than the 311.06 you have now? You might want to try 314.22. Drivers newer than that (the 326/327/331's) seem to have generated a lot of complaints from 500-series owners, but 314.22 was pretty well-respected.