How do you improve performance on Mercedes Benz 180E without causing engine failure?

The car is grossly underpowered, and mine suffers further because the transmission behaves weirdly. It thinks it can go up a hill in 4th gear at 41km/hr. Under light throttle, it changes up from 2nd to 3rd, holds 3rd for about half a second, then goes straight to 4th, but its nowhere near the

speed i want to cruise at.

Reprogramme the transmission NOT to use 4th gear at every speed above

40km/hr, if the throttle is not floored; to hold 2nd and 3rd until the "right"

speeds, what any driver of a manual would do.