Hi My 91 560 sel has the exact same problem that just developed in the last two months.. I have replaced the coil plugs wires cap rotor.. it idles a little rough at the moment still.. and when I give it gas it tries to go but eventually the power drops and I run the risk of stalling.. I give it power it bogs down and wants to stall.. I i let of the gas the revs come back up and the idle returns to mostly normal but again any attempt at acceleration leads to i mmdiate bog down and potential stalling.. update the ignitions system helped alot.. there was a time it would not even start..but thats taken care of.. Im thinking now its a fuel issue.. the pumps checked out ok.. the other components left are the fuel dist and/or the relay.. will replace the fuel relay/filter shortly and then the fuel distributor if those two dont do the trick...
good luck
Demeterial