...the check engine code(s)? Knowing that information is paramount to being able to troubleshoot this on your own. If you don't know the codes, I'd highly recommend purchasing your own OBD II reader...there are tons of inexpensive options that allow you to pair the reader with a smartphone using a Bluetooth connection and a number of free apps available. You plug the reader into your OBD II port (usually located somewhere under the dash) and pair it with your phone, and you should be able to see/clear codes. I would not clear any codes until you have made repairs. I think vatechcowpatty is on the right track. My first thought was vacuum leak...I'm not a Ford guy, so I don't know how that era Mustang is set up, but I do know that I once had a vacuum leak on my '01 Ram truck that played havoc with the HVAC system. The control that changes the airflow from floor to vent to defrost is controlled by engine vacuum, so depending on the engine RPMs and load, the vacuum would change and would randomly cause the HVAC air to start blowing out of different vents (i.e. - it would switch from the dash vents to the defrost vents or vice versa). I don't know if that type of issue would cause the AC compressor to shut off, but I do think that's a possibility, particularly if the hot/cold air blend door is actuated by engine vacuum, also. Otherwise, I suspect a refrigerant leak as vatechcowpatty suggested. Also, it would be good to verify whether the AC compressor is turning when you are experiencing a lack of cold air. The compressor is operated by a magnetic clutch that allows the compressor pulley to "freewheel" when the AC is not in use, so if the AC is set to "On" at the dash, you should see both the compressor pulley and the compressor clutch spinning together. Sometimes the magnetic mechanism can wear out and you can smack the clutch with a hammer/screwdriver and it will engage, but you have to be careful that whatever tool you use does not get snagged by the accessory belt in the process. As for the lack of power/acceleration, that could also be explained by a vacuum leak. If you want to try to troubleshoot that yourself, you can use a aerosol can of a flammable substance like carb/choke cleaner and spray around the intake manifold and vacuum hoses with the engine idle...if there is a vacuum leak, it will "suck" in some of the aerosol spray and should cause the engine speed to increase, which should be noticeable. Just be careful not to spray directly on something really hot like your exhaust manifolds or it could get a little bit exciting. Good luck...let us know if you/the mechanic are able to figure it out.