You need to give us more info. These would help. Tractor model, age of belt, drive belt or PTO belt, is it an OEM belt, how long is this been going on, age of the tractor, when was the idler pulley changed last. We are not standing next to you to see what's going on, and with 230 posts you should know that our ESP has been done for sometime now. Paint us a better picture.
With an OEM belt in good condition, a 30 degree angle may not be a problem. At that angle you should be able to see any wear or distortion where the pulley is mounted. In many cases not using a non-OEM belt will cause the problem, as will belt wear. Second, cause would be a worn pulley bearing which will cause the belt to ride incorrectly. Finally, is it routed correctly and is the tension spring pulling in the proper direction?
Rivets is correct But also look at the arm it is on. It could be bent and need to be streightened.