Kickdown is a combination of control pressure and electrical dumping of governor pressure. If you got shifting one moment and none the next, I give a big thumb's up to the kickdown switch suggestion. The easiest way to be sure is to disconnect the kickdown solenoid at the back of the transmisson. It just plugs in and can be reached from the rear by sticking ones hand to the passenger's side of the trans mount (this for the SL all other models its much easier) forward about 4 inches below and forward of the output flange above the pan on the right rear.
The reason for disconnecting it here is to totally eliminate the electrical activation. The electrical activation can also occur due to the B position switch and the malfuntion of the kickdown relay which is part of the klima or fuel pump relay.
It is most likely the kickdown switch itself and can be disconnected there. This seems to be more common on SLs for some reason.
Bosch Master, ASE Master, L1
33 years MB technician