70′ something 850 Case Dozer…losing power to the tracks conditionally

This was my last project child, as the title reads, it is a 70 something (plain) 850 case dozer withe the 301BD engine. Brand new undercarriage and (now) brand new rebuilt 301 engine. Here is the thing....I have pushed dirt for over 120 hours on this thing since the rebuild and it is a powerhouse for the size machine, If you push on flat ground, it is unstoppable...going UP-hill, the same....Now here is the kicker, if I get on an incline pushing on the side of a hill...(maybe a 3% grade)....it starts losing power to the tracks. The same thing happens when I push down a really steep hill and try to back out....(great power down...not so much backing up) As long as I am on flat ground or pushing UP-hill, I am good.... As soon as it starts losing power to the tracks on the incline, I come off the hill to lever ground for a min. and it re-gains all track power, and I hit it again... I am really stumped on this on. ALL...again I say ALL filters and screens have been checked and changed during the refurb. (throughout the complete machine)