It will all ways stop when you step on the throttle, your vacuum drops off when the throttle blade is opened, this is why they went to a electric wiper system.
Add some oil to the wiper motor, the old leather seals tend to dry out over time. Use a leather oil, I heard motor type oils will degrade leather over time.
If there is a question about vacuum, install a gauge at the wiper motor to monitor how much vacuum there is. Mine used to run like yours, slow and unsteady, I had them rebuilt and now they all most throw the wiper off the arm they are so fast. My engine idles with 19.5" of vacuum. They are easy to rebuild, but getting parts is tough. Joe
