Some that report this kind of issue find an issue with their belts as in they have lost their guage...ie the width of the belt has changed due to incorrect secondary clocking for the changes in loads or terrain they have been running in. It's easy enough to check. Just pop the cover, mic the width and compare to the spec in the service manual. Belts also stretch under prolonged loading and especially when very hot...and these do get hot. This also effects shiftout ratios.