Same problem I have...
I have posted numerous times with the same problem with my 560sl (also same diagnosis) but no joy so far. I have isolated mine down to one cylinder and an intermittent spark. Tried the Techron cleaner with no luck either. Mine is definitely not motor mounts. My expert mechanic cracked (loosened) the fuel to injector line on the bad cylinder and announced that it was a bad fuel distributor-unfortunately he is most likely right.
I would recommend pulling each spark plug and see if it looks normal (grey on the tip). My bad cylinder plug is coverered with black carbon (unburned gas I'm told). If any of your spark plugs look different (wet or black or broken), change plugs, check wires and/or pull the distributor cap and see if there is any damage or deposits, any cracks or loose wires. You might try running your car with the hood up in total darkness. If you see sparks coming off any of the spark plug wires, you need to replace the wire set.
I hope yours is something simple like a loose or bad wire-I don't think mine is.
Good luck. I'll be watching this post.