The following explanation is not mine, but out of my archives - on the WHY of the rotten egg smell:
"The sulphur smell from the exhaust, is actually caused by running the engine/cat convertor slightly lean for long periods and then running under a rich condition (ie going up a hill under heavy load) This is when the sulphur smell (rotten egg) is produced.
Under relatively lean conditions, the sulphur found in gasoline is converted to sulphur trioxide, then during the rich running condition the sulfur trioxide is converted into hydrogen sulphide (rotten egg smell) within the catalytic converter."
So, you can try different brands of gas, or check and see if your Forward O2 sensor is lazy. If you are not getting a MIL (CEL) light, it's probably only lazy (e.g not switching fast enough.) . I'm guessing gas and maybe the lazy front O2 sensor. Have the sensor looked at with a scanner and watch the switching rate.
Happy, and less smelly, Exploring
Chris