88 chev 305 chevy 1/2 ton problem

I have an 88 chev 305 2 whl drive,motor factry reblt 60,000 miles ago n tranny reblt 40,000 mi ago.I put in new distrib cap n wires, tps, egr valve, map sensor, oxygn sensor, fuel filter. All leaking exhaust components were fixed. I've sprayed around intake n throtle body with no change in motor idle. regular oil chng n tune up also.air filter and pcv valve chnged regular.whats happening is it starts off alright then it idles rough n my gas mileage goes very bad(10 miles a gal even less).tranny feels like its going in low gear. After I had done an install of new sensor or filters it behaved for a while then would go sour after a couple days.what happened one long trip was it climbed hill very slow I had to put to floor n it pushed out the dipstick and oil. sometimes it'll throw out oil from valve cover gaskts. I've tried turning key without starting to see if ses light will flash(it did). so ecm is probbly ok?I hate to trash this truck it handles so good .What can be wrong? I forgot to mention my master cylinder might be goin ,would an air leak in this area affect idling and shifting.(not likely but I'm not the expert) If it is your master cylinder that is bad, it would not be leaking vacuum. It would if it was your brake booster though. When the dipstick pushed out and poured oil out, this was the tranny right? There is a factory recall about this, I would have it fixed if it is not already.

Do you have an SES light on while driving? If so the computer is sencing that there is something wrong. To retreive the code, turn the key to run, with the engine not running, and use a paper clip to jumper the A