automatic gear box problem with 1993 W124 300 TD (280.000 km)

My automatic gearbox functions correctly when cold, in drivng gear and reverse. After some driving at motor temp of 80 degr C, the forward gears continues to function properly.
When shifting in rear, I have to wait for 3 or 4 seconds and then the car will move slowly; it seems if the rear gear is slipping, with almost no power available. I already have replaced the ATF oil and gear box oil filter. Is this a costly problem or can some adjustments be made to the gear box or gearbox servo?
Who can help ?
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I'm no expert, but I had a similar problem with my 87 300D (W124) only to find out the reverse clutch had disentergrated and eventually clogged up the valve body. I had to wait a few seconds for reverse, but it was solid then... this may help.
If the fluid had clutch material in it (brown color, IIRC) then you may need a rebuild. I got new clutches, seals, gaskets, and eventually a new valve body -- and I couldn't be happier (except for the brown ATF I took out thanks to the mechanic setting the modulator to have a 500rpm slip from the shop... sheesh)
If you're wondering, I didn't pick this mechanic... he was the dealer's choice. The guy did the job, and there's still a warranty, so I'm as happy as my $0 spent will get me.
Mike

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