Bad smell from exhaust on E30 BMW

First, let me apologize for the crossposting, but I think both groups are relevant and I read it frequently.

I drive an 1990 BMW 325is (E30) with 176 kmiles. About a year ago the cra was quite sluggish after struggling to get it smogged here in
CA. My mechanic found the catalytic converter to be clogged and the outside temperature sensor to the Motronic ECU to be lose making it run rich all the time. He put in a new cat as it was shot and clogged. Since then I replaced the muffler with another used unit that looked in good shape.

The last two or three months the car has started to smell quite bad,
I'd say and acid/sulphurous smell. This specially happens when I park the car after running it a while, especially at home since it's an enclosed place. I've researched the issue a bit, and it seems to be a somewhst common problem with rich running BMWs forcing the catalytic converter and eventually clogging it. I don't need to pass smog until next year but I'd rather take care of the issue. Tht car is running fine, and mileage has remained quite constant (~20 mpg with moslty city driving.)

I thought I'd try to see if I can do something about the problem myself. My mechanic (the one that installed the cat) closed shop last
April (hey this is Silicon Valley!) and reputable BMW independent mechanics are expensive ($90/hr!).

I thought of checking the whole cat/exhaust,a dn then check oxygen and temperature sensors. Any other ideas? I don't currently have easy access to a decent area to work on the car so I'm limited on what I can do.

If you're getting a sulfur smell from the exhaust, it's probably safe to say that the engine is running too rich. Checking the oxygen sensor signal to the ECU would be a good place to start, see if it's switching normally (note that it still has to be connected to the ECU as well when checking for proper switching..)

As you have found out they the Cat unit must get really hot. if not they clog and cause lots of problems. rotten egg smell. That Beemer is a costly little auto but
I guess you know this fact already.