check the spark plug wiring to make sure that the boots are sitting correctly on the spark plugs and have not come loose. there is also a part located behind the battery that is a black square of some sort (not really sure what it is, can someone help?) but when my engine was idling really high, if i disconnected it the idle died down to <500 rpm and sometimes felt as if the car would stall, so make sure that that is plugged in all the way or that you arent in need of a new one.