I have had similar issues and so first thing I did was switch to wider bars and that seemed to help a lot but I still had it from tome to time and the final conclusion was the straps on the pack… So with wider bars and a new pack (courtesy of the boss) the Osperey zealot 10, which has very thick and wide pads for the shoulder straps… i have rock a few packs in the last 3 years and the mule did help but not enough.. i have yet to have any issues with the new pack but i haven’t done anything longer than 2 1/2 hour rides with it..btw my bars on my full suspension are 31inches uncut and 28inches on my rigid niner… good luck