Proform Treadmill doesn't run constant speed.

Question

I have a treadmill that is about 10 years old but has had low usage.
It runs but when you are on the belt it slows down then speeds back up.
When you are not on it it stays constatnt speed.
I have checked the potentiometer slide control and it measures resistance correctly.
I'm guessiing maybe its the motor but how do I know for sure?
Its a PROFROM 375 SI