Republic Lagun KMC-250 eating drive belt belts

my republic lagun is eating belts.

just on 3rd one
1 1/2 hours of running continuous cutting a die and have a small bubble on outer edge of belt.

I am using 37 x 10 x 900 LI belts. mm

is this the correct belt?

today after running, I put my finger through one of the side port holes to check the temp and felt that the belt has quite a bit if slack in it when not running.
is this usual?

variable speed through a pulley drive system, operated manually.

any comments or ?????????? would be gratefully appreciated.

Thank you for your time.
Your efforts are appreciated.

Charles