Q: RPM issue. Car can't produce enough power. Idles low and caps at low RPM.

The car runs at very low RPMs and when in idle it sits with an inconsistent rev. It'll jump back and forth between 750 and 500 rpm (and when car it cold it'll drop lower or simply stall) and when given gas it will only rev to about 3000 rpm and essential cap itself (it'll stay there). At first I assumed it was a MAF sensor issue, so we replaced it, but it still is having the same problem. Spark plugs were replaced, sensors were cleaned, belt timings checked. I should mention that a catalytic converter did melt when this problem first started, that's where we assumed it might have been a problem with the MAF sensor and the might be running rich. The odd part to this is occasionally (believed once the car has warmed up a bit) after turning off and turning the car back on, the rev will work like normal. revving to 9000 like normal, but once you turn the car off and restart it, it will go back to it's state of low revs. Any suggestions would be greatly appreciated.

My car has 100000 miles.
My car has a manual transmission.