the extact thing happened to me with my 98 SVT. Interestingly enough the problem was related to my TPS sensor. Remove it and take it to a parts store, they almost always have a method to test the continuity of the resistance. If it is bad, normally resetting the battery and replacing it takes care of it. That what happened with mine. Also your IAC might be stuck. This is the valve that opens and shuts to basically choke your motor when uder a load (most notably when the ac kicks on and your engines spikes in revolutions). Best part, I wouldn't worry too much, this is an engine management problem, and not anything else, or at least that is what my problem was. Good luck.
