Jlg 2646E2 scissor lift questions please ??

Stumbled on this forum, and hopefully some one can help with a few questions.
Bought an old JLG 2646E2 scissor lift for home use. It seemed in pretty good shape. Got a new set of batteries and off we go.
All seems perfect, but when I drive the machine, if I encounter even a little lump on the concrete, it stalls. The drive/drives seem to have very little "power".
The machine lifts quickly, and everything else seems fine.
Any ideas on where to look first.
Thanks

Jlg 2646E2 scissor lift questions please ??