I think I've got huge, major $$$$$$ problems. When I accelerate it goes fine until I get between 3500 and 4000 rpm then it flat lines. The petal is to the floor and it acts like it's not getting fuel. Leaving off the accelerator , it shifts but does the same thing again which makes it very hard to merge with highway traffic. It is a 1999 C230 Kompressor. Since I got it I can't move the shifter lower than "D" but it ran fine prior to this problem. The veghicle has 90K miles. My mechanic had the shifter out and was able to move it through the gears but when he put it back in it still stops at "D". Is it shifter or transmission time? Everything else has been done to the car and when I get this done it will be 100%.

The shifter can cause a shifting problem, but your described symptom points to an engine problem. What does your mechanic say, any fault codes. I would diagnose for an engine malfunction first.

Just had the spark plugs replaces as well as the A/C condenser. "Check engine" light came on and mechanic found a loose plug as the code indicated a problem with #4 cylinder. Engine revs past 4000 rpm when vehicle is in "park" but not when accelerating into traffic. Otherwise it runs very smooth but it only gets past 60 mph when you do it slowly rather than trying to merge into freeway traffic or passing.