From your description, that belt is under a LOT of load if it is causing engine hesatation and slipping. Consider that something the belt it driving may be overloading the belt. (Power-Steering, Alternator, AC compressor....etc)My 2012 Outback has been having a squealing sound coupled with engine hesitation with AC on. I've been told by my mechanic this is normal and not to worry about it. However, it did it today three times with the AC off. Any suggestions? Thanks!