300E sluggish acceleration, with early gearshifts

[benzdriver737]

1989 W124 300E 280,000 km's has all of a sudden has lost its 'power'.
Starts easily but when I put my foot down hard on the accelerator to overtake, it refuses to kickdown and upshifts much earlier than normal. Won't go anywhere near the redline in terms of engine revs. Checked the transmission fluid, it seems ok. Tried unplugging the kickdown switch, no difference, same symptoms.
Recently changed the spark plugs and air filter, car seemed ok when I test drove after plug and filter change.
Today whilst the car was at idle with the transmission in Park, I could hear a metallic clunk noise coming from the transmission area. It wasn't constant, just once in a while, I couldnt tell if it was coming from the transmission or further forward near the engine, because it was only once in a while. Couldn't hear the noise while driving. Only seems to be there at idle with the transmission in Park.
Gearshift seems smooth enough, although it changes gears way too soon. Seems almost like gear slippage, although the engine is not over revving, in fact it won't allow itself to be driven hard.

Any thoughts/opinions...