I have a proform 740CS, it sometimes slows way down, if I step off it will sometimes speed up much faster than what it's set at, but then slows down again when I step back on. Other times, it remains extremely slow. Still other times, it works just fine. The speed readouts don't reflect the periodic slowdowns and speedups.

SOURCE: belt is slipping on a proform 740cs treadmill.

Similar solution on a ProForm 500i. Symptom was that the belt would slip whenever you'd step down.
Solution was to use an Allen wrench to tighten the bolts at the back (behind) of the running deck. Had to go two full turns. Careful to unplug first, and careful to tighten each exactly the same amount.
Also wiped the parts of the platform I could reach under the belt with vinegar water, since they were gunky.
These two things fixed the problem

SOURCE: Proform 740CS runs okay but the incline does not

ProForm Treadmills have a reset procedure for the incline function. With the key inserted in the console, activate the incline; while the incline is changing, remove the key. Wait 30 seconds, then replace the key. The incline should to maximum, then to minimum, resetting its limits. It worked for mine just 10 minutes ago; the procedure is in the manual if you have it. Good luck!