1996 Chevy Monte Carlo Idle Problems

I have had my car about a year, prior to me buying it, the previous owner had new head gaskets and intake gaskets put in the car at a auto shop.
Since it has warmed up outside, at idle it surges, varies a couple hundred rpms, and it will randomly rev up to around 2 or 3 thousand rpms at idle. When the car is driven, either through town, or down the highway, it has no real symptoms. It is giving me three codes, PO171: system too lean bank 1, P0300 Random/Multiple cylinder misfire detected, P0507 system RPM higher than expected. I have replaced the idle air control valve, mass air flow sensor, and cleaned the throttle body. I tested for vacume leaks by spraying carburator cleaner around every vacume line I could find. Im kinda running out of ideas. Does anyone have any ideas?