w140, M119, S500 - can't cure it

Hi everyone,

First of all, I read the forum all the time and did the seach - found many threads with similar problems but none ended up with a resolution. I am hoping that somebody might have found a cure and ended up not posting for some reason.
Problem #1. History: - the 1994 S500 had developed a rough idle (sort of "bumps", more pronounced when A/C is on), lack of power on low end, problems with acceleration - for example, if I accelerate slowly, not bringing RPMs up more than 2000 - it would accelerate fairly smoothly, if I push it harder it feels like I am running on 4 cylinders, the car shakes and rolls and after such event (pushing hard on gas pedal) the idle gets even worse, I would need to turn the car off and on again to reset it.
List of parts replaced:
1. Plugs and wires
2. Caps and rotors
3. AFM
4. Wiring harness
Where do I go from here?

Problem #2. The oil level sensor has developed a leak, quarter sized oil drops are visible on the driveway. It doesn't look trivial to replace it, as I need to get the oil pan removed and it required lifting up the engine. Am I right or wrong?

Thanks in advance.