I am having a weird idle problem with 94 e420

the idle jumps up and down erratically when in drive..if i switch to neutral and back to drive..idle is fine...then after driving some more..idle is erratic again..neutral..it is fine.

This happens after the car has warmed up.

I have replaced all plugs, plug wires, caps, rotors, air filters, wiring harness.

my car does not have ASR.

anything i should check or look for?

thanks in advance.

rashad