1999 Saab convertible idle

Have you checked the vacuum hoses and connections? Is this a Turbo?

Get a vacuum tester (MightyVac) from the auto department of your favorite Mart or auto parts store and check all the hose connections that go into the intake manifold. If the gauge doesn't show that the vacuum is maintained for at least a few seconds after a couple of pumps of the tester, trace that line for cracks or disconnects. The line may also be connected to a device that is leaking - like the Hooter Valve (turbo cutoff valve).

I'll go you one step easier Walt: Buy 6' of small vacuum hose. Replace all the small vacuum lines. They all dry out, especially the one that goes to the blow off valve (longest one). It will cost you less than $10, eliminate the issue if it's there, and they always need replacement by this age.