93 Ford t-bird misfire and rough idle

Hopefully someone can help. I have a 93 t-bird v6 3.8ltr. I put a new engine and transmission in it. I drives and runs great, seems to lack a little power though. At idle it sputters and eventually dies. I set the idle at 550rpm and the timing at 10 deg. BTC. I sit and watch with a timing light and for about 20 seconds the timing is good, then out of no where it jumps aroud and sputters, the timing goes from 20 BTC to 10 ATC back and forth then dies. If I rev it up to 800+ rpm then the timing goes strait to 10 btc and doesnt move. I put all new sensors, air idle control valve, throttle position sensor, cap and rotor, plugs and wires, even fuel pressure regulator. I am stumped and have dumped a few thousand into the car now. Please help. A friend tells me that since the engine is bigger now that the computer will need to be reprogrammed to compensate the new fuel air mix. But I thought O2 sensors did this? Please any advise to avoid taking it to ford. Thank you