1989 Cadillac Fleetwood Need Help Fast

I own a 1989 cadillac fleetwood fwd I dont know how to say how many miles it has but the last three numbers are 111. When I bought it from original owners it was running good. I dont know if this is the problem but right after I started adding sound system it started acting up on the highway. The accelerator was not responding for about half a minute then it kicked in. I ignored this and then came more problems, at complete stops the car would start shackin like crazy and trying to turn off, after a few seconds it would stop and be like nothing was wrong then start again. Thats when I started getting worried, I did a full tune up and no luck still the same. After a couple of weeks the car would act up while driving city street, the car would not accelerate it would just keep on rollin as if on neutral acompanied by shacking and loud poping explosions. Even the muffler was shattered because of these explosions. It would smell like unburned gas. From there on it has been mechanics here mechanics there and none of them have fixed the problem just taking my money. Ive payed for new fuel pump, distributor, distributor cap, engine computer, injectors, injector sensors, and I dont know what else. Me and my dad know about working on cars we just need to know the problem. The car has low miles and it is very good condition inside and outside original paint so please help me. Oh and I almost forgot there is no check engine light on or any light, and mechanics say there are no codes showing.