Round drive belt help needed

I have a Servo Products CNC mill with about a 1/8" thick round drive belt. I picked up a couple that are supposed to be for the machine. I went to measure speeds for various settings for future reference and was fine on the low range. On high range about halfway up the speed range the belt started coming off the pulley due to speed. It was still running in the grooves but getting much larger. A little higher speed and it was barely making contact with the spindle pulley and slipping terribly even with no load on the spindle. I ran through the speeds some time ago and don't recall this issue.

Should I be taking the belt off the pulleys when not in use so it doesn't get stretched out? I see McMaster Carr has lots of selection of these type belts in both neoprene and urethane. I am not sure the material of the current belt but it is black. Which would be better? The smallest pulley is about 1" diameter and goes about 20k RPM if the manufacturer claims are accurate. How do I measure the belt needed? Do I measure the relaxed belt length or the belt length when it is on the pulleys?

It may have used a round belt originally, or a previous user of your machine replaced the belt with an O-ring. That will work for a while, in a pinch, but as you've noticed, it's not a permanent fix. Measure the pulley diameters and their distance apart center-to-center and use a calculator (like this one: https://durabelt.com/beltlengthcalculator.php ) to determine the belt size you need. Don't go by the old belt, since it's stretched out already.

The belt is new and claimed to be proper by the supplier who also supports the machines. I will be doing some checking on the material of the belt. They claim that it is normal for it to stretch with age, environment, etc but haven't stated what the typical life is so it seems a bit suspect. It is only a couple years old and hasn't seen much running time. It looks like an oring but I don't know enough about drive belts to know if I could tell the difference between a common oring and a drive belt. Thanks for the durabelt link.

I was able to get the belt material scanned and it looks like EPDM. Most belts I have seen for sale are polyurethane. Not sure how each would work for drive belts but EPDM is often used for plumbing orings so it is looking more like I may have just a plumbing oring. I plan to get a polyurethane belt and try it.