2004 V6 3.9

Just had the clutch done a few weeks ago.. Maybe the shop forgot to tighten the exhaust completely? I've already seen some posts about it, basic things to check.. I cleaned the maf and the car idles exactly where it supposed to now except for when the engines cold RPMs drop to usually 500 nd bounce back and forth until it warms.. Car drives fine except at WOT.. Feels almost like a limiter kicking in.. Anybody else run into this? Next thing I'm going to do is have the vac system smoked and see if its leaking. Small evap system leak code came up a few months ago but cleared it and never came back, not sure if it could be related.. Thinking that's just the gas cap