new mass air sensor problems

[crazy5.0]

Last thursday I got a Pro-M 75mm mass air since my stock one was bad.My car finally has power again,but now my exhaust pops when Idling and letting off the gas.Also the car nows surges.I tried setting the timing higher and the car surged badly with the spout disconnected.While the car runs stronger now it still won't pull hard past 4000rpm.My exhaust started popping slightly about a couple months ago when the car had no power past 3000rpm,but now it's much worse with the mass air sensor.The sensor is calibrated for my injectors.My gas mileage still sucks at 11-12mpg.Does this sound like my stang is running rich?My plugs always look fine.Could I have a exhaust leak even though my vaccum gauge stays at a steady psi at 2000rpm?My o2 sensors are only a couple months old.

------------------
89gt,3.73's,tremec
3550,Steeda
tri-ax,aluminum
driveshaft,Asp
underdrives,Flowmast