i would guess the catalytic converter, wont always act sluggish either..if its coming thru ur vents its probably coming from outside of ur car...never heard of a heater or air conditioning part smelling like rotten eggs, usually that would be more like dirty socks...a bad battery or overcharging battery can also give that sulfur (rotten egg) smell <img src="/forums/images/graemlins/cheers.gif" alt="" />

Lonnie
Auto Parts Professional
95 sportage sold but not forgotten