A place for runners.
I don't own a treadmill but I have worked with belt conveyors!
My first guess would be belt tension.
It looks as if there are YouTube videos which show how to tension treadmills.
My only real advice is to make sure you pull the roller back evenly, otherwise you may get tracking issues (the belt wanders left or right).
Yeah tensioning is easy enough, There's just two bolts I need to adjust in parallel to move the back-roller. I'll see if adjusting belt tension fixes it.
Can't get my head around the greasing. Even though the manual tells you to do this I can't see how that grease wouldn't get transferred to the inside of the belt and then onto the rollers. This would then cause the belt to slip on the rollers giving what you describe.
It's specifically supposed to be on the inside of the belt to reduce friction between the board underneath the belt and the belt itself 🤷♂️
The driving roller is a nip-style mechanism, so the driving roller is pressing on the outside of the belt on the textured surface. I actually doubt the grease has an impact on the performance.
If you turn off everything else on that circuit, does it resolve (i.e. not getting enough power for some reason)?
That's not a thing...it draws whatever power it draws, if the breaker can't handle it, it "blows" and cuts off power to the entire circuit.
If power is an issue with the motor, it's within the treadmill itself, e.g. the driver for the motor is not outputting enough power. This is kind of unlikely when it's still functions properly otherwise. Most likely cause is something external that has changed, causing the required power to run the belt to increase (e.g. higher friction between board and belt than intended or wrong belt tension).
I worked tech support for years and even some laptops would have issues with various power strips or bad wiring. It seemed like a very simple thing to eliminate so I thought I'd mention it. Worst case, someone wastes 30 seconds. Best case, it highlights an issue. I live in a place with 100v mains now and have some American appliances and, in rare cases, it is noticable.
The you're also running with what is equivalent to a voltage drop of 20V, which is absolutely insane and would normally be cause for serious concern. Normal households don't see more than a few volts drop at most unless it's dangerously shoddy wiring.