Sniper,
It sounds like you have your EFIE set to high -- you are trying to lean your fuel too much. During the first half of the tank, the computer and EFIE are fighting and then finally the computer say, "OK, something is not right here. I'm getting strange and conflicting reading from my sensors, so I'm shutting down the fuel management system and going to a set overly rich mixture to prevent any damage."
You will need to 'Play' with that EFIE setting -- Find the spot where it almost turns on the check engine light.
1991 Plymouth Acclaim 3L V6.
1 dry cells with nineteen 6"x8" 316L ss plates, driven by constant current PWM set at 35 amps (13.3V at PWM). 28% KOH electrolyte. Total measured output 2.5 lpm. Mileage went from 18 to 26 mpg, all city driving (44% increase). EFIE set at .370 and I still need to play with ignition timing.