Hi guys. My new to me 1990 560SEC has a surging issue while going down the freeway. It is more noticeable the faster it goes, starting at about 65-70mph and getting worse at 80 or 90.

When I push on the throttle it accelerates more than it should, and when I lift it drops faster than it should. Making it hard to modulate.

Worse on cold days. It has been in the 30's lately and it is more marked then.

It feels almost like a wandering vacuum advance, a fuel pressure issue or sticky injectors. Is there something that needs cleaning? A sticky solenoid?

Plugs/wires are good, rotor/cap looks OK. I have run about 1/2g of sea foam thru it, and a quart of mystery oil.

Thanks!