Those are the symptoms of "bad drivers" or "failing texture units" in you 550ti-- ( a hardware fault).
Its better to use Nvidia proprietary drivers as they more capable.
I had the same issue with my crossfire 6790 config until the card would artifact then go on to distort the color of the entire screen...much to my inspection one card was faulty and got it RMA'ed.
The same problem in windows would manifest as a BSOD under GPU strain.