My 420SEL has a rough idle

[retroguybilly]

The idle on my '87 420SEL is rough, but it smooths right out around 1,000 RPM and drives fine at speed. The engine seems to have enough power, but there is a slight hesitation from throttle tip-in to part throttle, and after that it starts to run strong.

I've replaced all the obvious stuff: cap, rotor, wires and plugs.

What else would you check if you were me?

Thanks a lot!