I had a similar problem with my 90 300E. It would occasionally stall and idled roughly. I took it to a European specialized repair shop to have it tuned, distributor changed, new spark plugs and wires. I was disappointed when I received it back with the rough idle and occasional stalling continuing.
Fiddling around, I was looking under the hood. To the right front, behind the headlight, I found what appeared to be some sort of smog or air mixture devices. Looking closer, I found the vacuum lines to be cracked and aged/weathered. I bought 4mm vacuum hose and replaced each end (less than a foot of hose). From there on, it starts much better, rough idle is gone, no stalling, and accellerates very nicely now.
It's worth a try to look and very inexpensive fix. Hope this helps.