Starting/stalling/backfireing/bad idle

Ok where to start I have a 1990 olds cutlass sup. With a 2.3 HO out of a 92 achiva with about 30,000 on the new motor. Now the problem I have been haveing truble with idle mainly. It will idle up and down a bit in netural/park but when I put in gear it will stall after a few min running unless I keep my foot on the gass a bit. I also have truble starting it. It will go to start but in mid turnover it will stop and if I do it a few times it will back fire on me. Also when I have it running my rpm gauge which is a digtial read out will flicker all the way up a few times and when it does I can feel the car shutter a bit then it will die. I have had this car in and out of the shop and no one can tell me what it is. I have spent way to much on this car to just give up. I just repalced the head gasket 6 months ago and have had this trouble every since then and a bit before. But before the head gasket went the idle was way to high. I have replaced tps, map, O2, IAC, new igiton coil, and igiton control modual. And still have all this trouble. I was just told that it might be timing or a Torque Converter Lock Up Solenoid what ever that is. So if u can give me any help let me know or have any more?S just ask