I've had this 1994 C280 for the last 6 months and it runs very smooth, changing gears at it supposed, at around 3000 rpm. Lately, I noticed that with no reason it changes from 4th to 3rd gear (at around 45-50 mph), causing the engine to rev, and it won't change back to 4th. If I put it to neutral and then to D, it take over from the 4th, as it should.

Any ideas on what may cause this problem and how much it would cost to fix?
Mileage is some 124k and I know from the previous owner that tranny fluid was changed some 10k miles ago...

Thanks for the expert advice.

 

...SLEEPWALKER ??!... please...!!