Or rather:
I had same problem with my 1991 S300 (European W140 - similar to US
1992 model).
Bad air flow meter caused too lean mixture, causing catalyst to
overheat and melt down, i.e. the ceramic structure pulverized and was
blown into the muffler system, again causing this to block.
With the bad air flow meter the car in this case would run with a
little less power and quite some hessitation in accelleration. But
whith the blown catalyst and blocked exhaust system, there was almost
no power left to get above 35 mph.
Repair cost at a dealer was 5.000$ including new air flow meter,
catalyst, oxygen sensor and complete exhaust system.
But then it runs beatifully again.
Your problem could be the same.
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