My '96 Saab 900S has multiple symptoms, and I suspect multiple problems. For one thing, it gets the error code:
P0422 Main Catalyst Efficiency Below Threshold (Bank 1)
I made a video of my vacuum test results. The idle speed and vacuum pressure tend to fluctuate mildly, and if the A/C is on, the idle speed oscillates 200-300 RPM and the vacuum drops below normal. Intake leak maybe? Or exhaust blockage?
After holding the throttle at a higher RPM for a few seconds, the needle goes crazy! Bad slip rings, I suspect. But there's also this ticking sound, which doesn't change with the speed.
So my best theory is a combination of bad piston rings and either an intake leak or exhaust blockage (bad cat?). I'd appreciate any advice as to what to check next, or tips from folks who have seen this sort of problem before.