Some background before the problem started, one day out the blue i turn my Evo Iv on in the morning and all of a sudden the base idle was at 2000 - 3000rpm instead of 1000rpm which would then correct it self after the car was warmed up and sit back at 1000. it done this for awhile, i cleaned the throttle out not that it was dirty. made no difference. then a few days later it was magically back to normal and idleing better than it ever has making me scratch my head even more.

Few days later it has started reving up and down at idle between 1000 - 1500 rpm while cold and corrects itself once warm (unless i drive it around under high boost and WOT it will do it while warm also). Took it for a drive and all was fine in first gear no issues what so ever. but once i change up into another gear (or shift down a gear) and i put my foot down heavy the rpm will shoot up for a second with out the car accelerating correctly (almost like the clutch slipping except the clutch has been tested and is healthy) then correct themselves and be fine in that gear from then on, it only does it if i put my foot down heavy and it only does it once per a gear. can only describe it like its almost purging air out or something for a second then it will run perfectly fine in that gear until i change up or down a gear.

So basically at the moment i have to change gears, put my foot down to make it rev up for a second and then it runs fine, i can WOT from any rpm range and it wont do it again until i change up or down a gear. (note it doesn't matter if i drive slowly for 5mins after changing in that gear then put my foot down or 2 seconds it will still rev up and not accelerate properly for a second once per a gear). Also there appears to be some misfire or vibration at peak boost but its not running any codes

I also recently replaced a fuel line that was leaking

below are a quick snippet of the mods, car is also tuned to just under 400hp

 

I would start with a boost leak test. Then clean the MAF if you are still running one, check the plugs, maybe remove negative cable on the battery to reset the ECU etc. Start with the simple stuff and try and isolate from there if it continues. But if the engine can free rev while in gear with the clutch engaged, thats deff a clutch issue.

 

Mitsu used to use an idle switch. My '92 had it, I don't know when it went away. When the switch contacts got dirty the result was a 2000 rpm idle - a bit disconcerting in traffic. The fix was a new switch.

If the engine is actually revving but not accelerating it has to be the clutch.