TRANSMISSION STRANGE BEHAVIOR
 

I have 1993 300SE. Recently transmission started to exhibit a very strange behavior. When I put shifter on D, the gear engages than disengages for a split second, than engages again and still engaged. It becomes more pronounced with warming up, it still disengaged somewhat longer. After a city driving for 30-40 minutes, after stop at red light, it happens that the engine is revving like in neutral, the car is standing still and finally the gear engages only after 10-15 seconds. What can it be?

Have you checked the fluid level?
Has the fluid & FILTER been changed lately?

Sorry, forgot to mention fluid level is OK and it's crystal clear. That's the first thing I'm checking. Fluid and filter has been changed something like 12,000 - 15,000 miles ago with Red Line synthetic ATF, adding when necessary Mobil 1 synthetic ATF; how many years I don't remember.

Sounds like internal trans issue unless the filter is plugged OR the trans cooler is blocked.

I would suggest a filter change.

I changed filter and fluid already. Transmission works somewhat better, but double shifting jerk is still there. By the way, I put additional trans cooler from Summit 5-6 years ago, nothing special, just U-shaped aluminum tube with fins. I put it between transmission and radiator. Should I remove it?

Make sure fluid can easily flow thru it.

If you are in a cold climate, remove it. The transmission cooler in the radiator helps to warm the transmission somewhat, as well as cooling it, and unless you're towing more than the car is rated for, the stock trans cooler should be adequate.

Other than that, sounds like an internal pressure issue, start by testing and adjusting the pressures per FSM.