I had two separate problems with two different sets of symptoms and causes :

On 260E:
1. a) Engine was OK at high speed but would stall at traffic lights.
b) Engine was ok when cold but once it got to the operating temperature rpm would drop and eventually stop. However when I kept the RPM at about 2500 it would run smooth.
c) Hard start after stalling

These problems disappeared when I took the Ignition Coil out of my 300E and put it on the 260E. (of course all these symptoms then appeared on the 300E)

On 300E:
2. a) dimming lights at low RPM
b) Seemingly rough idle - (as if jumping a cylinder)
c) hunting engine
d) fluctuating Voltage at the battery

These symptoms went away when I replaced the rectifier/regulator on top of the alternator.

__________________
Zorin

'88 Benz 420 SEL
'79 Porsche 911SC ('87 Carrera Engine)
'00 BMW 540 Sports Package
'99 Land Rover Discovery

86 300E - Manual (sold)
88 260E (sold)
84 944 (sold)