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Usually it's leaking seals in the actuators. Parts are not expensive and it is a relatively easy thing to fix. Here is a vacuum trouble shooting guide.
Gary
I agree with Duke94. It is probably seals in the actuators.
This is assuming you have good vacuum to start with. You don't have "lumpy" engine that makes little vacuum at low speeds do you?
Have you checked for vacuum leaks - loose hose, etc. Also check condition of vacuum hoses for cracks.
The headlights operate off a vacuum. If you don't have good vacuum they will be slow to operate or won't open at all.
I don't know what year C3 you have but you might want to replace the check valve and filter on the vacuum hose running from the intake to the vacuum reservoir tank.