I had the same problem with the unit not going into high speed most of the time and sometimes when turning. Found the handle pivot bushings were worn just enough that the microswitches at the pivot area would click out when pulling the handle to turn.

I've found that these machines are very sensitive to battery condition so I made up a adapter cord to plug the unit into another battery pack to check if it is a battery problem or not. It has saved me a lot of time, instead of checking out the batteries when the problem is with the unit itself.