I have a 1991 300E2.6 that I purchased used with 36000 miles and now is at 99000. Never realized the performsnce was lackluster until I drove another (same model/year). I had the CO reset which helped but with the onset of 100 degree weather the car is a pig after it warms up (the car runs well in the winter). It's worse with the A/C on. Cylinder pressures are OK. A vacuum guage attached to the intake manifold shows a flutter, big drop then slow rise with any throttle greater than 1/4 throttle. I found no vaccuum leak on the intake side. Replaced the O2 sensor. I've heard it may be the Catalytic Cobnverter. How can I check that without spending $500.00 to replace it?

It is unlikely that the cat is at fault. Warm weather will make some cars slow because the ignition control unit will retard the timing. You can try replacing the temp sensor on the rear of the cyl head.

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