vent motor or resistor???=-

98 lumina, 3.1
about 265,000 miles.
vent motor has been changed about 75000 miles ago(??)
I have the usual vacuum reservoir problem.but if I play with the speed switch, etc. usually it blows out the correct vents.

question= sometimes the fan volicity seems weak, then later much stronger.
is it possible that,s a sign of a resistor malfunction? or just the motor going bad ?

thanks for any advice

First, fix the vacuum reservoir. That way all the air comes out the vents you select instead of defaulting to the defrost position.

I'd be testing the voltage on the power wire to the blower motor. Given a current probe and an oscilloscope, I'd be testing the amperage draw of the blower motor, which with some observation of the 'scope and a hint of mathematics will also get you the motor RPM.