I was having this problem on one of my grinders the other day.I found out that my drive wheel had wore some and was a little out of line and making the belt run off funny.I also had this happen once when my platten had wore some.
If this is the problem it ois easy to fix by taking the wheel down to a machine shop and having them dress the face or drill the center hole out and put a shime in so it will fit the motor shaft.
Just some of my fixes for this problem,
It iws a grizzly the drive wheel is the contact wheel
it is good shape not that much has been ground on it
I may try the tape on the idler wheel
if you have anymore input I woud\ld be greatful
Try some different brands of belts. I've found some crawl all over the wheel with even moderate pressure while others stayed right in place. Generally, I've found the cheaper the belt, the more it moves around. Your milage may vary.