[Solved} Yet another erratic idle question.
 

85' 928s, been running fine, daily driver, recently it will randomly jump up to very high rpm when shifting to park or neutral after driving. Like under 1k rpm up to 3k rpm. It's dramatic. When I put it back in drive it lowers to 1k, which is still higher than normal which is about 750 rpm.

Any ideas what I should investigate?

​I have all new fuel lines, regulators, dampers. My last idle problem was cured, I thought, by replacing last damper.

has the ISV been replaced its wear item.
also check the throttle cable for smooth operation,
and make sure the throttle quadrant push rod is not getting stuck on the lower radiator hose clamp

Thanks. ISV has not been replaced. I was thinking it was cable issue too. Should cables or pivot be greased?

Make sure all the intake boots between the side plenums, intake runners and large T to throttle body are in good shape. They tend to split on the under side where it isn't obvious.

replace the ISV now,
if your old one is sticking it can damage the computer diodes
lubricate all balls and pivots check the cables for smooth operation lube as necessary or replace

Replaced air valve. When I first started car you tachometer signal but eventually it came back.

Update. Its been a few days and car is doing good. Probably should have replaced that "wear part" last year. Thanks Mr. Merlin and Roger.