buttafingaz

I recently purchased a 1985 Mustang GT 5.0 Fuel Injected. This car only has 67,000 original miles as it was hardly driven. It sat for a lot years and has recently been brought back to life. Now I have it up and running and seems to be good except random idle jumping. I have heard it can be a vacuum leak and I am troubleshooting this as we speak (well tomorrow in the daytime I am). I just have a question about the Dashpot and it's function. Now I read this controls the throttle if the engine RPMs drop rapidly. If there is no vacuum leak could the dashpot be the culprit? I have noticed if just sitting at idle the plunger on the dashpot almost seems stuck.

Now for symptoms.
When I am sitting tinkering with it the idle will never fluctuate will run like a top. When I am driving it I will notice the RPMs will jump up to about 2000 and stay there forcing me to pull over as this is unsafe. I will manually adjust the idle and will be good for a while. Now at another random interval the idle will now drop down to where it should have been before but since I kicked it down (via screw) it will stall a lot. So now I adjust the idle back up and all will be well for a while until it will kick up again. Can the dashpot get stuck and keep the idle higher or does this simply sound like a vacuum related problem?

Things I have replaced to get her back up and running.
Transmission filter
Transmission fluid
Oil
Oil filter
Plugs
Wires
PCV valve
Air filter
Cap
Rotor
AOD bushing was gone and replaced for proper shifting.
Ran some octane booster and fuel injection cleaner and a few full tanks of super unleaded through her.

Any advise would be greatly appreciated as I am not a mechanic I can just wing it most of the time.