Battery or controller?

[drandle]

Took the Tomberlin Vanish down the back yard last night to move some rocks. Back yard is on a hill, not too bad, i've had it down there many times. Drove fine down the hill. I put it in 4wd and tried to back up to where the rock is but it only moved in very short 1 second spurts with about 5 seconds between the spurts, even at full throttle and back in 2wd. Finally got it back up on flat land and it drove fine. Should also state that the battery meter showed 3/4 full.

Any ideas?? Controller or batteries? or ???

TIA,
Don