My hunch in explaning the faster speed is that with your momentary loss of composure, you accidently had the belt on the wrong pulley sheave. You might consider something more permanent than a piece of toilet paper to keep the pulley sheave locked onto the pulley shaft. As torque is applied at each start up, that paper binder will likely loosen up and start to slip again. Maybe some kind of glue or cement that's compatible with plastic and adheres to metal.
Glad you finally found the problem. All's well that ends well.
Tom
Glad you finally found the problem. All's well that ends well.
Tom