Just had a new problem start w/my 95 E320 coupe. It has about 181K miles on it. When I'm in 4th gear and I apply heavy throttle - RPM spins up w/o increasing road speed. I tried this in 3rd gear and it seems to work ok in 3rd w/o slippage - even when I apply heavy throttle.

I recently (about 2 months ago) changed my xmission fluid (drained torque converter, dropped pan and replaced filter & pan gasket). I refilled using Mobil 1 synthetic ATF + a pint of Trans-X conditioner. Transmission ran fine up until today when it began to slip under heavy throttle in 4th gear. It also flares when upshifting under heavy throttle. I checked trans fluid level and it looks ok - fluid is pink and doesn't seem to smell like burnt clutches.

Any ideas of what the problem might be?