1) A long shot but if the card has any minor revisional differences other than (and possibly including) the GDDR5, the driver may glitch just enough now and then.
2) No judgment b/c I do this too sometimes, but if the card is used (say, from the bay or otherwise) it may have just enough usage, or led a less than perfect life, or just naturally begin to fail early.
3) The card, whether used or NOS, may have less than ideal thermal compound under the heatsink. Youtube may heat up the card in a way that desktop usage does not. Also, verify (for sure) that the fan on this card is spinning properly.
4) I'm reaching here, but while 512MB is fine for 2D HD resolution and several monitors of it, modern Youtube may be pushing across the blurry line into a more 3D-like accelerated universe. These older 512MB cards MAY lack the necessary acceleration features to not heat up significantly, and/or may lack enough 2D frame buffer once it's split 4 ways. I don't know if it can be dynamically reassigned, although I imagine it could.
5) Windows Update. (Btw, what Windows is this? I'm gonna guess 7.) Graphics driver update. [I hope not, as old as these cards are. Ask BlueIris users about recent Intel drivers and their memory leaks.
] Other updates? Enough clean wall power to the whole project? PSU aging? (edit - formatting)
6) Monitor things with GPUZ, HWMonitor 1.33, HwInfo64, etc, and see (just for fun) what's going with temps, voltages, buffer and memory pressure (if reported like on newest cards), powercap reasons, all that stuff.
7) The Newegg reviews for the GDDR5 version start on July 1st, 2010. You know what I'm going to say next...right?