A long shot but known to be a problem is the intake manifold gasket. These start to leak for some reason and all sorts of weird things will take place since the air/fuel ratio is always trying to correct itself.

A quick and actually safe way to find such a leak is to pick up a tool that looks like a long thin pipe on the end of a hose. The other end of the hose connects to a propane tank like the propane torches use. Start the engine and crack the valve on the propane tank so that a very small amount of propane is coming out.

Then wave the end of the pipe with the propane feed close to the intake manifold bolts and runners. If you hear an increase or decrease in RPM you have found a leak where the engine is sucking in air and, in this case, propane. Check all the vacuum connections in the same way. Any change in RPM is a leak because the additional propane screws up your air/fuel ratio.

Mechanics do this all the time. It sounds risky but I don't think you would have any problem with doing this in a safe manner. This also has the advantage of being repeatable. You engine may change RPM at odd times but with this method every time you move the pipe to the same spot the RPM will change.