Vacuum Advance sticking?

What would cause the vacuum advance to stick when the engine is revved? This is a Mopar Performance electronic distributor and if you rev the engine over about 2300 rpm the engine will come back to approx. 2000 rpm and stay there until you disconnect the vacuum line from the advance. You can plug the vacuum lines and the car will return right back to the proper idle speed. I`ve noticed one of the springs(the larger ,silver one) on the advance is very loose. Any help out there?