Serious Fuel Problem?

Ive had my 05 GT since Feb. 8. Since then, its been in the shop for everything, bellhousing, shaker, popping noise, and the fuel tank most recently. Im telling ya, they didnt do much work on my mustang, everything that could have possibly came messed up, did. But anyways, ive been getting real crappy gas mileage, about 12.7 and im 75highway/25city . So it should be alot better. There is also something very wrong with my computer when it comes to the fuel. Today I put $5 into the tank that had 37 Miles to E. I turn the car back on after putting in the gas, and it did not even move, the gas didnt even register. Ive also had situations where the guy cant get the gas to go and then when it goes to $1 it just starts takin the gas fine, like gas is just disappearing into the abys. Ive let the car sit for about 10 min, maybe to get it to settle... it worked once but not tonight. Do I need to do something or should I go back to the dealer? this is annoying and its costin me alot of money $$$$ Im a dam college student paying off my mustang, the last thing I need is to be wasting money on invisible gas.

I have JLT CAI with 91 Octane Tune if that helps if anyone has had the same problem
thanks
-mike

Oh no, Im not spending money on getting the things fixed Im spending money on the gas that isnt registering or behaving wierd. Thats whats pissing me off.
-mike

Thanks guys for all the responses, i think i figured out the problem.... I put a fuel like substance in the fuel tank to like clean it out and now that im looking back that seems to be the problem. Gonna try that stuff you guys suggested.

Power Surge, yea I had my fuel tank replaced already... maybe i need another?
-mike