I don't have any gasser MBs, but I had similar issues with a Nissan with a crapped-up EGR that got stuck partially open. It ran fine on the freeway but bogged at lights and had like 1/3 of the power- so the mileage was terrible. I also had an intermittent issue with spark plug wires like lsmalley in a chevy- when the motor warmed up- the boot would work loose, causing an intermittent miss- and it would ground out on the header. Took months to find that one. Had to run it- pull the boots off one by one (and noticing the change in idle) until I noticed NO CHANGE when I pulled #7. I forced #7 boot back on, and it sprang to life- and idle went up- and then shocked the p*ss out of me because the exhaust header had burned a tiny hole in the insulator- which was arcing on the header... Best of luck.