1990 Hyundai Sonata

My car barely wants to go, it was getting worse and worse and all of a suddon stopped driving past 5 m.P.H. It will start to go up to 5 m.P.H and if you push on the gas to get it to go faster, it will start to slow down. It won't rev up either.

I already just replaced: fuel pressure regulator, and already checked the erg valve. I just replaced the fuel filter, and checked the trans fluid. Nothing seems to work and every mechanic I have asked has no clue.