TBI Chevy van spark advance

I'm working on a 1991 G20 chevy conversion van with a 305 auto. It will start fine, but idle very roughly unless you hold the gas pedal down just a tad. Once it warms up, it will go through phases of smooth and rough idle (in rhythm, as if only one or two cylinders aren't firing). It will run alright up to about half throttle, then choke and die, as if it isn't getting any spark advance past about 2000 rpm (no tach). The ECM is throwing codes 43, and 44 (ESC and lean mix). We've tried changing the MAP and TPS sensors, as wells as the EGR and spark plugs. I tested the wires, and the cap and rotor look fine (once the carbon was cleaned up, which didn't help any). The EGR was full of carbon and wouldn't move by hand, and the TPS tested bad according to alldata's test procedure (over 1.25V output with the ignition on but not running). The low vacum to MAP sensor code went away after changing the MAP sensor. Changing the MAP, TPS, and EGR helped smooth it out, but it still won't stay running.

Any ideas?

sounds like a bad fuel pump