Sounds like an old shift modulator but it could also be a rotted rubber connector tube between the plastic vaccum line from the engine and the modulator.
Someone posted about this about three or four months ago and said the difference was amazing.
Check your vaccum lines and their connectors.
Modulators do go bad.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)