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?