certain brands of fuel can also cause a rotten egg smell.
The easy way to check for a clogged cat is to pull the 1st O2 sensor and see what kind of pressure you have and see how the car runs with it out.

Any chance of an intermittent coil? Do these cars have 1 small cat per side like many of the GM v8s?

BTW. I've seen it where worn engines loose compression as they warm up. Most likely due to cold thick oil helping the worn rings keep compression but washing away as it gets hot.