Run OK Cold, Sputters, Backfires when Hot. HELP!!

Hi, I have a 92 E-150 club wagon with the 5.0 liter V8

It runs ok when cold, when it warms up it will idle ok and you can rev it up and it seems normal.
When you put it in drive and give it some gas it sputters & will backfire a little.
If you accelerate with youf foot almost off the pedal it will go, but not very fast.
If you "pump" the gas it will go a little better, once you get it over 40mph you can get it up to highway speed.

CLUES:
Complete engine rebuild 1 1/2 years ago.
Replaced fuel pump about the same time. (NAPA)
Drove it about 40miles to my home in December - Ran Great but tranny slipped in low.(2, d, od were fine)

Parked It about 6 weeks
started it to charge battery, ran out of gas.
Put gas in it started and ran, although a little rough.

Parked it about 4 more weeks
Replaced transmission.

It now has the problem!

Replaced fuel filter.
Replaced Throttle Position Sensor.
Removed and checked EGR Valve-Good!

ANY IDEAS?

Who would have guessed this...
OIL IN THE GAS TANK.
Not some obscure problem, vandelism.

I went to siphon some gas out of the tank,
and my tubing kept coming out of the filler tube oily.
So I dropped the tank, sure enough oil in the bottom.
Drained the tank and wiped the inside dry (BIG FUN!)
I also cleaned the filler tube & cleaned the fuel filter with
spray carb. cleaner.
Runs good again!