Am I the first to post the reason why bad motor mounts cause RPM drop, rough idle, and stalling?
The knock sensor picks up vibrations in a very specific range and sends a signal to the EEC, which in turn pulls out ignition timing and if the engine is at idle, it will cause the engine RPM to drop and possibly stall especially in gear.
Actually, a friend of mine discovered this and told me about it. His SC had the rough idle, interior vibration, stalling problem and could not figure out what was happening. He finally had a mechanic hook up a monitor and shift the car from park to reverse to see why the RPMs would suddenly dropped. He saw the EEC was pulling a lot of timing out for no apparent reason. They heard something knocking around and guessed a bad motor mounts were causing contact between the mount bracket and oil pan to the cross member which inturn was picked up by the knock sensor. They replaced the mounts and the problem was fixed.