I find that the transmission is generally slow to kick down, but it is especially slow at higher speeds. Is this normal? I also am experiencing what appears to be irregular fuel pressure. At times it is sluggish, and the back pressure on the accelerator is noticably less. At other times, it is very responsive. Changing the fuel filter and using fuel system cleaners has helped, but it remains an intermittent problem. What should I try next?

I'm not a tech, so this is just my opinion.

I would think you have to replace the kick down switch, get the tranny adjusted, or have it checked for any vacuum leaks if it is like the tranny in my 1992 300CE.

Have you had your air filter changed recently as well, if you havent, it might be plugged slightly and causing the car to need to struggle to breathe in air.

Perhaps the accelerator cable needs to be lubricated.

Hope that helps, but I would wait for one of the pro's to answer this, I know quite a bit from reading this forum over the last several months, but I am by no means a pro, or even a tech...

Alon