Hi there. I am going to be giving you all the details. I have a 88 firebird 5.0 tbi. About a month ago the rear main seal started leaking real bad and the valve temp seals were gone in the heads because the exhaust was blown smoke like no tomaro. But not much driving problems. Then out of no where one day while driving it started going bad. The exhaust started poping and then the car was jerking and sounded like it was missing. I then noticed that the rpm gauge was going haywire jus jumping up n down. I wasn't sure what the problem was so I drove it to my buddy house were his dad is a mechanic and I was already planning on buying a rebuilt 350 from his shop. So I take it there and he jus decides to swap motors first n see if it helps. So we did that 92'suburban block350 . 187 casting # off a 89 tbi I believe . Bored .060 over. With stock 350 1.94 I believe intake valve? and I believe stock 350 sized exhaust valves. Basically just everything same as a 350 head except 305 compression chambers. Port polished them up real nice as well. Mild cam I'm not sure how much of a performance cam it is but it's smog legal. Edel Brock intake and stock tbi on top. Some jeg headers. Now we swapped it in last nite and everything was set ready to go jus needed to time the engine. Now he cranks it over and smack!!! It has the same damn problem now I was watching the timing light and it seemed like it would miss at a time? I'm not sure because some times it will idle his really rough but then he was jus adjusting the timing and the idle screw. We have not took it for a drive because it dies and it I'd not timed. Pleas help! What sensor could have for bad to cause this? Maybe a relay or something idk??? Please I jus put 1400 in for this motor and swap and is so disappointing to see this happen. Thank you all who help!