I think I saw this topic before but here it goes again now that it's happening to my car. So here's the deal, recently whenever I use the A/C or Defrost mode on my HVAC system, the car will suddenly feel like it surging forward kinda like a rough idle. So I'm thinking since it only happens on defrost and AC mode it has something to do with the compressor. It never happens when I just have it on vent (hot and cool). Any help would be appreciated!!! BTW, it's on my 1990 3.0 Bull.