John Deere 750 Dozer Transmission problem

I have a newly acquired 1978 JD 750 (E or B?) Dozer. After warming up to operating temp in a hard push the transmission pressure gauge will drop into the yellow and it quits pulling and just chatters.....the engine is in great shape with only 1000 hrs since major....undercarriage is in great shape.....it is worse if on a hillside vs flat ground....it doesn't always do it and only when pushing hard. As long as pressure stays up it is fine....Any ideas? Thanks, Jerry

John Deere 750 Dozer Transmission problem