I have had the same thing happen with my 97 S420. It is very simple, and has nothing to do with the transmission wearing down. If you were to remove the wood trim around the Gear selector and window buttons etc. you will see a white plastic module on the right side of the gear selector box. Un-clip the module and you will see two copper strips. The strips are supposed to touch each other when the trans is in drive, any time you move the gear selector lower than drive, they will untouch and the transmission will only drive up to 3d gear. Here's the catch. These modules are very defective. When going over the slightest bumps, these two copper strips can void, creating a situation where they are not touching. What i did, i took the copper strip that was in the back of the module that has a hook on it, and pulled the hook forward and over the front copper strip, this way they are always touching. This will solve the problem. You can still move the trans gear lever into a lower gear after this. This is definately a do it yourselfer project. Just be careful when removing the wood trim. (Wrap a cloth around a flat head screw driver and insert on the side of the trim, and pry upwards. Start prying up in the back.
I hade a similar problem with my 600 sel. the car has 2 air filters. one of the tubes from the air filters had gotten disconnected causing 1/2 of the engine not to get all the air it needed. this caused jumping in the rpm. If i were you i would check all the hose connections around your air filters.
after running your car for a while hold your hand over each air filter and often you will feel one side is hotter. this is a easy way to tell which side is not working.
-hope this helpe -kristian
My 1994 S320 suffered this for a while. The car was sluggish and felt like it wasnâ€™t firing on all cylinders. The idle RPMs were erratic.
I opened the spark plug cover and replaced the plugs and re-seated the wiring on the HT leads. It looked like one of the wires leading to cylinders number 6 and 2 were slightly frayed. So I guessed it could have been arcing onto the engine block. I taped this up with black insulation tape and I no longer have the problem. The ultimate fix would be to replace the HT wiring loom - This I will do at the next service interval.
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