I had that problem with an MSI 4400 replaced my power supply as I found the +5v was over 10% below 5v and I think they recommend it to be nearer 5%. Still had problems and took the card back and they agreed it was faulty. I was told it is usually caused by the type of memory chips. The care they replaced it with has heatsinks on the memory maybe you should try to get one of them if neither of the two you have tried had heatsinks.
Suggest you also check that one of your power supplys stats is not too high or low as it might be your power supply that needs replacing.