I have an electric Yale forklift from 1977, it jumps 2 feet when I hit the throttle and then nothing. Earlier it has been working fine, but for some while now it have been comming and going. Now it is jumping each time. Forward and reverse works fine and also the forks. What I can say is that earlier on low speed it has been quite weak and there have been a change in weather from -5C to -15C. So I thought it could be something with the bateries, but they are fully charged and shall be in good condition. I am in desperate need of help, the forklift need to be in working condition within next week.

I have made this video to descrive the issue:
http://www.youtube.com/watch?v=Gc1Qp-MTcyQ&feature=youtu.be