Does the treadmill run ok at those speeds if you aren't walking on it?
(i.e. if you let it run on its own)
Does the underside of the belt appear worn or frayed?
You may need to add some belt lubricant to keep it running smoothly:
The belt lubricant should be applied to the underside of the belt.
This helps the belt to run smoothly, however if the underside is frayed or worn it will need to be replaced as a permanent solution. To see underneath it, you will need to loosen it.
A tight belt or worn, sticking belt would cause it to stop like you describe.
If the speed seems to be adjusting by itself, then one of the buttons may be stuck on the console - perhaps the Speed- (decrease) button or the lowest speed setting number, if your particular model has those buttons.
Try pressing and releasing the buttons several times to see if they appear to be responding normally, or if they might be sticking down.
Let me check if the other techs have any further ideas...
You may have overtightened the belt... Its a common issue, when the belt starts to slow they tighten the belt instead of lubbing the deck... Then when they do lub the deck they forget to loosen the belt again and it puts strain on the motor....
Loosen the belt off a little and be generous with the lub... As you loosen try and fix that off center belt at the same time... If your adjustments are at fullen tighten then you certainly have too much tension on the belt, loosening off will give you more play to center the belt again...