940 Non-Turbo

I have this irritating idling issue that has not gone away since I've first posted here several months back.

Perhaps I may have missed the obvious so would some one tell me if my car's symptons is more a vacuum issue or is it due to some failing sensors.

Symptons:
When cold the rpm needle would go down to less than 500 at times almost stalling the car. It does not stay low but climbs back up as sudden as it drops. It will probably do this for two to three cyles before steadied up at 700-900, alway hunting but never remain steady. The situation is worst if I either step on the brakes or put the gear in reverse or drive.

Car would hesistate when engine has not attained operating temp. Once warmed up the car runs okay (not very good with no so good acceleration) and at traffic stops the rpm needle continues to hunt between 700-900 and intermittently drop below the 700 line.

It does not surge like going up to 1200 but mostly dropping in rpm and moving back up to 900 and down.

Is this a vacuum problem because I want to have a more focus search. I have checked hoses and could not find any leak but maybe hoses are beginning to get porous with air seeping through the rubber. Maybe the manifold gasket is also leaking although I could not hear any hissing sound.

Please can someone give me a clue from the symptoms I've just mentioned?

Also can a out of specs valve shim cause this?

If vacuum is the likely cause I could begin with replacing all hoses, one at a time eventhough I do not spot a leak that is visible upon examinination

Thanks all!

laozong