A/C intermittent

Have the freon charge checked.

You can also bypass the low pressure switch to see if it a low pressure problem yourself.

Not a good idea to run it bypassed for any length of time tbough.
 
Run it at idle and watch the compressor. If it only clicks on very quickly and then kicks off for awhile then it's low on gas. It's cheap to just try and put a can in there. A Mustang won't take more gas then it needs, so don't worry about over filling it.

Kurt