JLG 2646e scissor lift driving slow, but speeds up when steering

I have a JLG 2646e scissor lift. When trying to move it forward or backwards it almost has no power to do it, and is super slow, but as soon as I try to steer it either way, it speeds up and drives the speed you would expect. But as soon as you straightennthe wheels out, it loses power to drive straight forwards or backwards. The lifting up operates well. Any ideas ? Thank you