I have a 1991 300E2.6 that has POOR/Deayed Acceleration. The car has 101,000 miles, but has had the problem since 36,000. I've been told it is a clogged cat. The car is a pig it the summer regardless of the engine coolant temp unless the car has been stopped and idling (like at a stop light). My mom's old 240D with 5 passengers heading uphill would outrun unless I engage 1st gear and wait the 3 seconds between giving it gas and the car accelerating.
A running vacuum test shows 15psi at idle with a slight cycle of 1 psi at idle that becomes a steady 15psi at 2000rpm and at 3000rpm. The vacuum shows a very slight increase with rpm stable. Vacuum jumps to 20-22psi as I retard rpm and stays at the higher setting if I stabilize the rpm again.
Any advancement of the throttle (rapid or slow) produces a drop-off to near zero with a very slow rise "normal".
The car was tested in park (no-load) and driving (loaded) with similar results. With the A/C on all numbers were lower and it was slower recovering from throttle advancement.
Any ideas what to check next. The car tends to run at 95-100 degrees C in the summer with peaks near 110 when idling.

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