Idle/rpm Surge And Dying

New here so I'll start off with Hello. Back to business. I'm not a mechanic obviously since I'm here asking for help on a forum. I don't have money to throw at a mechanic I don't know or trust. I'll start from the beginning. Bought the car back in 2014 and drove it around a bit Its a 1990 LX convertible 5.0. We had overdrive go out so we tossed an extra AoD we had in it. It had sat for awhile and when it was drove is was surging badly Say 3000 RPM to 1000 and so fourth. We parked it again and its been sitting till recently. It also will not idle without some feathering of the throttle left on its own it'll surge from around 1000-500rpm and die. We replaced the IAC and EGR from our other mustang cleaned the PVC valve back in 2014 and didn't fix the problem. I also checked and set the TPS sensor to 0.98. Wondering if anyone has had any issues like this and shine some light on this or give me a check list to go through so I don't replace parts that don't need replacing. Thanks folks