If the IAC solenoid is failing cleaning won't help. With the engine running disconnect the IAC solenoid harness. If the idle falls or the engine stalls the IAC is good. If there is no change in idle the IAC solenoid is defective and the IAC needs to be replaced. This is a no cost test that you should do before throwing parts at it. If you hadn't said you replaced the MAF I would have suggested you clean it first. Test the IAC as I stated and report back.
Correct on the cleaning part, but a stock or mostly stock Mustang should idle just fine with the IAC unplugged. The only exception is if the A/C is on, which puts more of a drain on the engine at idle. I believe the original purpose of the IAC is for the A/C running at idle, but I don't know for sure.
Copied the Idle adjustment from another thread (and removed the TPS adjustment which has no relevance on our cars):
Try resetting your idle with the following procedure. If your idle problem is NOT being caused by a bad part or vacuum leak then I have never seen this not fix the problem!
Begin with a cold vehicle. The idea here is to get the car to a firm cold idle with enough air bleed capacity left in the idle circuit for IAC adjustment.
The idle stop should be set first. Back out the idle stop screw, away from the bell crank arm, until about 1/2 turn past the point where it no longer makes contact (blade fully closed). Using a 0.010" feeler gauge, tighten until gauge just drags between screw and bell crank arm. Remove feeler gauge. Tighten screw exactly 1 1/2 turns. If the screw is very loose, put a drop of loc-tite or silicone on it, so it doesn't work out of adjustment.
Now remove the connector to the Idle Air Controller (IAC) just on the other side of the throttle body. Start the car and allow vehicle to warm for 2 minutes. Give a small "blip" to let it settle. If it is having a hard time staying running you may have to get an assistant until you can get to the front of the car. Now open or close the air bleed screw (CCW opens) next to the IAC until the car idles at 575 to 600 rpm. For guys with aftermarket cams and an EEC tuner, you might want to idle a bit more briskly, say 650 to 675.
Obviously, this rpm range is by what the car and driver wants...IE, no set idle speed, whatever works for YOU.
Turn off the car. Now count the number of turns clockwise to close on the idle air bleed screw. If it falls between 1/2 and 2, it's okay, now reverse it out the same number of turns. Log the number somewhere in case you need it for the future. Reconnect the IAC. You are done.
If the air bleed screw is above 2 turns, it's a good idea to tighten the idle stop screw another 1/2 turn, and then repeat the idle setting. If it is below 1/2 turn, then loosen the idle stop screw by 1/4 to 1/2 a turn, and repeat the idle setting. Be sure to put another drop of silicone RTV on the stop screw if it was disturbed. Reconnect the IAC.
sometimes cuts off. i cleaned iac and got a new mass air new air filter and tuned out the egr. it still does it. and sometimes it wwill randomly sstart up and idle fine and rn great then if u cut it off it will run horrible sometimes and sputter and lag and it will go away.i checked some vaccuum lines i have no idea wat it is 
