WEIRD SURGING ACCELERATION!!!HELP!!!!

[Euphoricglass]

Ok so i bought this 91 se-r that just rolled over to the 100,000 mile mark..... but its had an infinity G20 sr20de put in it recently. When i start it in the morning it goes right to fast idle and warms up with no misses or flaws. Once it gets warm the idle gets a little erratic jumping from 800 to 600 to 700 then itll even out and miss a little at idle. When im coming up to a light and have to stop ill push the clutch in and itll drop down pretty low sometimes to like 400 then come back up, that could be from the light flywheel and underdrive pully though......... its got a pretty hellashious exhaust leak and when i give it more than like a quarter throttle it sounds like its surging though(only under load) u can barely feel it in the car( its not real jerky) who knows it could just be the resonance of the exhaust leak i dont know. its also getting pretty piss poor gas milage. i just changed the plugs that look like they had never been changed. Any help would be awesome.Im not a noob but this is my first SE-R

[PapaSmurf2k3]

Where is the exhaust leak? It might be messing with your O2 sensor causing an erratic air/fuel ratio. It also might be worthwhile to seafoam the engine... your idle air control valve could be crapped up.