Had the same issue with my 96E300d, and I started with the TPS, cleaned the contact plug and reconnected. In another thread, a poster brought out the fact that there are two vacuum pressure switches adjacent to the air filter box on the left side of the engine compartment. They are both the same part number, and one controls the egr valve, and the other the resonance flaps control switch. I removed the electircal connectors to each, cleaned them and reconnected both. The third item mentioned is the small vacuum switch on drivers side of the engine compartment that is attached to a small hose coming off the intake manifold. Again, I removed the connector, cleaned it, and reconnected. I also tested all the vacuum lines themselves to insure that there were not leaks. It has been almost a month since I did this procedure, and code P0120 has not reappeared. It should be noted, that even with the code, and prior to doing the work outlined above, the car showed no adverse drivability issues whatsoever. Hope yours turns out to be as easy a fix as mine did.

__________________
96 E300d