My 1988 300SE suddenly developed a problem with the automatic transmission a few days ago. When the transmission attempts to upshift into 4th gear, the shift takes about 2-3 seconds. During this time the engine revs freely and the transmission feels as if it was in neutral. When 4th finally engages, it happens very sluggishly. All other gears engage normally and kickdown and reverse are also OK. To make things more difficult, the problem occurs only occasionally. I've noticed, though, that it is more likely to happen when the engine is cold (it's below freezing here) and/or when the revs are over 3000.

The car has 65000 miles on it and has been regularly serviced by the dealer and has not been abused, so wear and tear should be out of the question.

I have very limited knowledge about the way automatics work, but I understand that the one on my car is controlled by vacuum. Could a vacuum leak be the reason for this kind of behavior since the problem occurs when there is less vacuum in the manifold? (As far as I can tell by the "Economy" gage)

Any ideas and insights would be greatly appreciated.

TIA
Jari