1995 Mercedes Benz S500 hesitation / fuel pressure / stalli

Engine Performance problem
1995 Mercedes Benz S500 V8 Wheel Drive Type unknown Automatic 90,500 miles

I have been having trouble with my car hesitating when I accelerate - even on the highway when I am putting continual pressure on the accelerator, as well as city streets. The check engine light was on, however, after a mechanic reset something (?) The light went off. It is now back on. On occasion, the car will stall when at a stop light (it seems like it only stalls after the car has been driven for 10 or 15 miles - or more). While at a stop, if you keep giving the car gas - while in neutral - it won't die. I have had it to the car dealer on two occasions as well as to a reputable mercedes mechanic. The fuel pump has been cleaned and then replaced, the fuel tank emptied and cleaned, the screen from fuel tank to pump cleaned. After the mechanic took it to the dealer to put it on their computer it showed that the wiring harness needed to be replaced. It was replaced as well, but it is still having the same problems. It has been suggested that there may be a problem with air in the tank, not enough pressure allowing a steady flow of gas, a faulty fuel injection system, and just about anything else having to do with the fuel system. It has also been suggested that the fuel cap be replaced or to have the ignition checked. The iginition was supposed to have been checked by the dealer about 6 months ago with no problem identified. I had the car to the dealer because my key would stick in the ignition for a few seconds (and eventually having to wait about 10 seconds) after the car was turned off. The car is used mainly for short trips (20-30 miles a day) with an occasional long trip to WI (about 900 miles). The mechanic indicated that he has done all he knows to do. We have spent over $2500 checking out this hesitation problem and the problem is not resolved. I don't doubt that the parts that have been replaced were needed - but I don't know what else to do or where to take the car. I have not been satisfied with the local dealer and the mechanic is now at a loss as to what to do. The next closest dealer is 100 miles away - and although I don't have a problem taking it there, I cannot be without a car for long. In additon to suggestions on the possible causes for the problems, is it necessary to take the car to a mercedes dealer or mercedes specialist to have these problems checked and/or repaired? Also. If the car is having these problems, is it wise to keep putting money into the car to repair things or should I look into trading it in? Please help! I would be grateful for any suggestions for resolving this problem. Thanks.