I have read and learned a bit on this website. I think that I should share my expensive experience with you guys.
My car is 94 S420, I have exactly the same problem as you describe... the engine is on but no power to the transmission, ASR light lit up, turn off the engine and start normally as it never happens. This happens just once in a while, it only happens when I apply brake especially when it rains. I bring my car to change a harness... it costs me CAN $1300.00... Just seconds after I leave the shop.... it happens exactly the same... Holly cow... So... I change the brake light switch ($25.)... And ... the problem is solved. I believe that this switch is not only light up the brake light and release the shift gear solenoid but also send the signal to the computer to dis-engage the engine from the transmission. To test this, I put my car on Drive gear, one foot on the brake and the other on the gas pedal, the car does not force to go forward. which means that when I apply brake, the engine and transmission is actually dis-engaged.
I hope that this helps and makes sense to you

Cars I had
1991 560SEL
1994 S420