Before you put additives in, make sure you do the following first:

1. Drain transmission fluid (including from the torque convertor) and replace filter & pan gasket
2. Check that transmission fluid is at *correct* level (use the search function if you're not absolutely sure how to do this)

#2 above is the most critical. My 300SDL started doing exactly what your car did and it was less than 1/2" low on fluid. I changed the fluid (I highly recommend Mobil 1 ATF) and set it to the proper level. That was 30,000 miles ago and it has been working fine since.

Neal

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'83 300SD 335,000km (207k) mi SOLD
'87 560SL 163,000km (101k mi) SOLD
'86 300SDL 356,000km (220k mi) SOLD
'92 500SEL 250,000km (155k mi) SOLD
'90 300SL 140,000km (87k mi) SOLD
'01 S430 260,000km (161k mi) SOLD
'03 SL500 167,000km (104k mi) SOLD
'07 S550 4MATIC 235,000km (146k mi) SOLD
'07 GL320 CDI 4MATIC 348,000km (215k mi)
'13 GL350 BlueTec 4MATIC 170,000km (105k mi)
'14 SL550 72,000km (43k mi)