Hello, My van is a 305 G20 series

Hello, My van is a 305 G20 series. I have replaced fuel pump, fuel filter, 02 sensor, tps, EGR, MAP sensor, I put a new IAC in but the idle still stayed low, so I put back the old one. The throttle body had an complete overhall, new gaskets, O-Rings, filter bucket, everything, and I had also replaced the base gasket. There are no vacume leaks on top of the motor, I tested every hose with a can of carburator cleaner. But my idle goes irratic, at times during the day, from a bit high, to a very low. But the van seems to have power, it runs smooth. (New plugs, wires also) but the gas tank is completely rotten inside, And the sending unit is rotten, wires carroded on the inside of caseing. I am in process of building a new tank, sending unit and pump. There is a code that comes out TPS too low. On a 1988, I dont believe the tps can be adjusted. It just screws in. I had to a small rubber bumper used for furniture on it, to lift it alittle, but still the same. Plugs are burning well, motor inside is pretty clean, for its year. I'm sorry I wasnt able to clarify this in my previous question. Could you shed some light on this problem, I will be indeded to you. Wayne