Upload & Sell: Off
I tried a low level format and the light stayed normal till I had maybe 50 files on it or so and then it started the low blinking again. Haven't had time to try the card much in one of my other bodies, so that is next.
That does sound like the card. Try a different card.
If you have a pc, do a surface scan on the card. It will read and write to every location and tell you if there are bad cells. This is a long test, but thorough.