Mysterious Problem Baffles My Mechanic

I have a 1995 S500 2Dr Coupe with 44K miles. If I do not drive the car for approx. 1 week, it starts right up and runs fine BUT 15 minutes later the engine starts to idle rough
and eventually stalls. If I try to immediately restart the car the starter kicks in fine BUT the car engine will not turn over and it BACK FIRES with a putrid sulpher cloud. If I wait approx. 30 minutes after it stalls it will start right up an runs perfectly as if nothing ever happened. If I drive the car daily, or every other day, it runs perfectly. The problem ONLY occurs when I let the car sit for 1 week. It is VERY predictable and occurs like clock work. My mechanic thinks its a bad sensor. He hooked up test box to computer port in engine while it was idling rough and it showed NO problems. He changed the thermostat sensor but that did not help. HELP !!

[vinamg]

How's your idle? You might have a bad cruise/throttle actuator. Try revving the engine and let go. The tach should drop straight back to spec 700rpm. If it goes below for a moment and comes back up you have a bad actuator.

Vinamg

Thanks for the input guys....
I forgot to mention that the gas filter was changed also to no avail. Oil is changed very often with Valvoline - no additives. Exhaust system for contamination? Hmmm I'll have to ask about that one. I'll take a closer look at idle....but when the car is running fine...it does not drop below 700. When its running like crap I believe it does...I'll have to take a second look.

[Ashman]

Where are you located?

If in los angeles area, I can recommend a good mech for you to go to.

Alon

[h2odiesel]

If we're making wild guesses I'd take a close look at the O2 sensor. They can start to get lazy and cause problems and not set a code. Ask your mechanic to monitor the cylce interval on the O2 and see if it a little slow going form rich to lean. Or you can just throw one in and see how it goes if it has been a few years it wouldn't hurt anyway.