First Mercedes,

After following jassz's adventure, which sounds somewhat similar, I would have someone press on the gas pedal while you have your head under the hood observing the linkage. The unit on the firewall is a common failure point on older cars, and so are a few other bits of the linkage, especially those with rubber or plastic parts.

Look for a change in action at the pump compared to the movement of the shaft connected to the pedal, as the pedal is depressed and released. When these stop moving in a 1:1 ratio, something is amiss. The problems most often occur where the linear motion is converted to rotational motion, and then back to linear motion. Most of the time a problem with the linear motion is either a bent link that enables contact with something nearby or an obstruction (something has broken off and fallen in the way) that prevents further motion.

I will also relate a problem my father in law had with the tank vent. His car would actually bog down and stop, then restart ok and run for a while. He became very confused though when the reserve light would come on with the tank nearly full, while this odd behaviour was going on. Then, when he tried to open the fuel filler cap, it was stuck on tight. He has hands like vice grips and got the cap off, and noted a whoosh of air going into the tank. Once the cap was off, or put on loose, the car ran fine. He found the rubber hose connectors going to the vent box in the trunk, and then out the passenger side fender behind the bumper, were soft and gooey, and kind of sealed themselves off. He replaced them and the problem went away for good.

Good luck, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)