My vacuum wipers acted similarly when I 1st started using them.
I took the vacuum motor out and squirted about 2 tablespoons of 10w30 engine oil into the port where the vacuum line connected then shook the vacuum motor at every conceivable angle to make sure that oil covered every square inch inside that motor. I then remounted it and it works pretty good for vacuum wipers that is
Keep in mind that anytime you accelerate engine vacuum drops hense your wipers will slow to the point of just stopping in mid wipe. Let up on the gas and they'll go to town again
Driving a vehicle w/vacuum wipers in the rain is a science all to itself
Good luck
Dave
