Low vacuum can be an issue on cars with aftermarket cams. What you are seeing may be normal. Did the heat come back when you let of the gas?
Does your car have the vacuum reservoir under the hood? That is supposed to help with that. Two things are more likely than a bad actuator:
1. The check valve in the reservoir is bad and lets vacuum bleed back to the engine.
2. You have a small vacuum leak somewhere and the reservoir can’t hold a vacuum while the engine experiences low vacuum during high rpm or acceleration. The leak could be in the actuator.
If the actuator is working, that is probably not it. I don’t believe they can be serviced.
If you have a vacuum gauge, tee it in to the small vacuum line that goes through the firewall & watch it when you turn the engine off. That may tell the story.