Problem on my 91 PA

Hi,

I've got a big problem on my Park Avenue.

When I take it on morning, everything is perfect. On start the idle speed is around 1400 rpm but goes quickly on a very stable 600 rpm. The engine is smooth and powerfull.

But if I stop (for a few minutes or hours !!!) and restart it, the behaviour is completely changed:

The engine is noisy, the tachometer goes first around 1000 rpm and after a few minutes begins to oscilate on idle (between 550 and 800 rpm). The oil pressure is a bit lower and sometimes oscilate too. The car miss power: I have to push harder on the throttle pedal to drive it. And I frequently hear a "pump" noise (the same noise I hear when I start A/C).

IAC valve, fuel filter and spark plugs and wires have been changed. Air filter and fuel pump have been checked, but without improvements.

What could be the cause of my problem ?

Thanks a lot ...

Did you checked the PCV valve and the Oxygen sensor?

Did you checked the PCV valve and the Oxygen sensor?

Oxygen sensor has been changed 3 months ago. But PCV valve is two years old. I will check it.

Many Thanks.