High rpm when idle and surging/stuttering

so I have been having this problem and haven't
been able to find anything that fixes it. recently for
no reason my Mitsubishi has been idling high when
starting and when stopping (like at a red light) as
well as stuttering. like the car physically surges. it
also drops im rpm when using ac ehich also
causes stuttering. I'm not sure what the problem
could be. I've read that it could be spark plugs,
coils, transmission, air filter, the catalytic converter  
or even the engine. My main concern is that i don't
want to take it to a mechanic and have them do
work charge me for things I don't need. if anyone
can help me narrow down what my problem might
be it would be greatly appreciated.