When weight is put on belt it stops. works fine by itself at all speeds

Is the belt correctly tensioned. Do you feel that the belt is slipping. Does the motor noise change and slow down, or does the motor sound the same? If it slows, then the motor is failing, or you have exceeded it's weight capacity. If the motor remains at the same speed, then the belt is slipping with you as the resistance on it.

The machine is struggling under load, it’s either a really badly damaged running belt with no lubrications. Or, a motor on the way out. Or, you are exceeding the weight limit of the treadmill.

Try to clean the dust inside the running belt and silicone them after.

Then try to increase the belt tension. all treadmills have these adjustment.

If it still slip, then problem with the motor.

In my eyes there are two kinds, ones that are a real grandfather clock that runs on gravity using a series of weights and gears with different counterweights for chimes and running the clock itself, and there are the show pieces that look like a grandfather clock but run on newer tech and electronics. If a real one then would try first pulling the weights to the top. They are usually shaped into a pinecone or something. If weight is at bottom them have to pull corresponding chain with weight till the weight is at the top of case. As "time" goes by the weights lower. You should have different counterweights for the clock, chimes, dancing figures and whatever else it may have. All the weights move at different "speeds" so you have to keep an eye on them to make sure it keeps correct time. I would recommend consulting the manufacturer if possible. There are so many kinds out there is really hard to tell by such a short video.

Loose is not the answer. Proper lubrication and correct tension adjustment is the answer. I don't know how many HP the motor is for, but for 265 pds it should be at least 3HP.
Put the speed at about 3MpH and while walking, make a small jump on the belt to ensure that it is properly tensioned. Needless to say that the belt must be well lubricated. If you see that the belt tends to stop when you make the jump, turn both allen screws exactly the same amount clockwise. Half a turn might be enough. When you're satisfied and the belt doesn't seem to stop, give both allen screws another half turn clockwise.
All that as long as the treadmill is capable of handling your weight.

George A Faria,
I understand that the turntable stops turning and then the motor shuts off. I also see that you're stating the appliance works fine without the belt on it. My suggestion would be not to run the dryer without the belt. This can cause wear on the motor itself. What I would suggest is to put the belt back on. If it still stops turning and the motor is shutting off, I suggest contacting either the manufacturer or a local authorized technician. This way the root of the issue will be found and your dryer will last longer.

-Best Regards-

the belt in side the hood may be too tight. or the tsc may be set too low so take a small flat head after you remove hood and on the mother board should be an adjuster turn it while tread mill is safley position and turn it slightly

What make/model is the treadmill? What speed does this behavior occur? Does the walking belt slip, while the motor and rollers continue to turn? If the walking belt is slipping then the motor belt needs to be tightened. If the walking belt slows with your weight on it, does this happen at 1-3 mph and still occur at 4-5 mph? It could be your body weight causing it to slowdown, try to increase the speed to 4 or 5 and see if it still happens. If so, is the treadmill rated for your body weight? Please be more specific.

check drive belt probably slipping /stretched out and needs replacement