Running problems caused by AC?

Well my Fiance went up to Wyoming to visit her family(about 400-550 miles put on that weekend). On her return trip she was pulling of the interstate to get gas and it stalled.... She was able to start it up and go...but while she was there a mechanic(self-proclaimed) said he had one and you can't run the AC for long periods because of lack of cooling in the jeeps. Fiance turned on the heater let it blow out and it ran greeat with AC on the rest of the way...... When she got home we checked the CE light. PO420 Engine catalyst below threshold. Well light has been cleared and problem hasn't returned.... But is this common? It has a new water pump, radiator, and t-stat. Would a larger electric fan help this problem.. what else can I do to cool it other than hood vents?

Thanks again!

Engine Catalyst - wouldnt that have something to do with the cat converter? possibly the O2 sensor that sits AFTER the CC?

Well it says it is a running problem caused by vacuum leaks, bad fuel, or just the fact it got hot and died from what I was told!

How hot did it get, if she can recall?

It has no temp guage...Just a light.. the light did not come on.. I am going to add a guage ASAP now....

I see I see.