ProForm 1200 ZLT - belt randomly runs slow

Hi, I have recently bought a cheap second-hand ProForm 1200 ZLT treadmill. It doesn't appear to have been very well looked after but I have tightened up the running belt and hoped that it would suit my needs. I am now getting an issue with it where randomly the belt will run at a very low speed regardless of the speed setting I have set and when I stand on the belt it stops. As mentioned I have tightened the running belt and I have checked the drive belt which does not appear to be loose or slipping. Does anyone have any ideas on what the issue might be and am I able to salvage the machine?

Here is a video of the issue - IMG_2397

Here are some videos of when the machine is running normally at various speeds:
IMG_2391
IMG_2385
IMG_2392