Battery Full [Q] Phone Incorrectly Reads Battery Level

Hey all, I'm in need if some advice/help. My Galaxy S3 seems to be incorrectly gauging how much battery life it has.. For a better explanation, here's a familiar scenario: phone is 100% and will slowly keep creeping down by a percent with little to no use. In a few hours it "dies." If I leave it for a couple of minutes or do something like take the battery out however, it'll have a reading of 36%. I can do this several times in a row! Another instance is say it was at 100% and drops down to 80% in an hour or so. If I leave it in standby the battery level will actually start creeping back up after a couple of minutes.

I have tried these steps and suggestions to combat this problem:
  • flash to CyanogenMod
  • buy an extended battery
  • calibrate the battery with apps
  • wipe battery stats

I've run out of ideas short of bringing the phone in to T-Mobile and have it replaced. Google searching has turned up pretty fruitless so I'm asking here before I make anymore moves! Thanks for all the anticipated help.