JD2010, Kubota3450,2550, Mahindra 7520 w FEL w Skid Steer QC w/Tilt Tatch, & BH, BX1500
Couple maybes : Maybe just running short on power rather than losing it. Are you in a low gear or ... Is it HST drive? The climb just adds to the work. If you go too fast it can exceed your engines ability.
Maybe you have a fuel starvation issue - from filter or other blockage that will not allow your engine to develop full power.