I am having the same phantom downshift syndrome on my 92 300CE, 114k miles as John H. noted I can drive for several hundred miles and then bang, it will suddenly downshift to 3rd gear. Once it ever freewheeled before downshifting but only once. It will downshift into 2nd gear on rare occasion.
I have serviced the transmission, no garbage was found in pan and, after breaking the metal case open, the filter was clean. I checked the detent / pressure cable for a sticking cable and found it to be free and it returned well.I adjusted the accelerator cable so that most of the slack was gone.
On Wednesday the car drove just fine and on Thursday the car acted up both in the morning and the evening commutes. I stuck my foot into the gas and when I let off it would upshift just to return to phantom downshifting within a mile or so. after 3-4 miles of this it was fine for the rest of the ride home.
I am going to back off the pressure adjustment and see it that does any good.
What is the"T" behind the nickel plug that John H noted is this the modulator pressure adjustment?
I understand that there may be a vacuum switch on the side of the transmission that can go bad but have not found any info either ya or nay on this.

Thanks for your help in advance

P.S. I sold the 93 Sportline as all three were a drain on the insurance budget.

__________________
Jeff (moose)

92 500E 90K miles stock less AMG chip

73 Datsun 240Z, 74 260Z, and a 75 280Z 2+2