with the battery pretty much fully charged, but run around for 10 minutes [to remove what I call the "surface charge], what is the battery voltage, both at rest, (with no load on the electrical system), and under hydraulic stall for 3 seconds, (tilt back until the cylinder dead-ends, then hold the hydraulic tilt control handle back 3 more seconds). a difference of more than 10% indicates a battery that would cause the effect you describe. the battery does not have cohones to work the ramp any more...
the hydraulic stall test puts _much_ more load on the battery for this test than driving, even up a ramp.