I was driving my 1987 300 E and all of a sudden it seems to rev high but not match the speed. i.e. it seems like the transmission slips at high speed, however the transmission seems ok as in it engages well and does not jerk, Also it was rebuilt about 18 months ago. Could it be anything else, note it does not happen always. If there is too much transmission fluid could this happen? There seems to be about 3/4 quart more than there should be.

Thanks