Help...Snapper belt jumps off drive pulley up onto drive shaft?

I have a 2 year old self propelled walk behind 21" steel deck HIVAC.

Model # P2167520B

I replaced the drive belt 3 lawn mowings ago with a OEM replacement 'stretch" belt hoping that would solve the problem...it hasn't. It's acting like it gets heated up and stretches and pops off the drive pulley....always up onto the shaft. A PITA to keep putting back on in the middle of a yard cutting.

Can the engine shaft pulley be moved up the drive shaft up little to get better alignment with the driven disc?

How would I do that? Knock out the key, loosen set screw and move it?

There is no way to adjust the belt that I can see....this may be my only hope...thanks

Robert
April 2013