stman,
ok so there is not a premium display on this truck judging from your answers, just a bdi / hourmeter (standard curtis 840 display)

still this standard display should show battery charge, hours and also will display faults when they occur

The ESC/FA communicates with both premium and standard displays and shows faults when they occur.
So once again i ask, when this problem occurs do any codes display on the display? When you are ON the truck trying to drive it. If yes tell me what numbers you see pop up.
If nothing comes up on the display then you can also go into the control system using the multifunction handle and see what error codes are in the history. You can also clear any history and make any adjustments to the system you need.
The only thing to doing this is you NEED TO BE EXPERIENCED working within this control menu. DEFINITELY NOT for the novice, and you WILL NEED the service manual.
As far as knowing Yale trucks? I worked for them for almost 18yrs... i think i know their trucks fairly well ;o)

My question to you is are you a forklift tech? or just an end user with some mechanical experience trying to 'do it yourself' ????
If the latter then you have a long road ahead of you to learn this machine. Go to your Yale dealer and buy a service manual.
If you need a quicker fix then call them out to fix it, they can hook up to the controller and see what is actually going on with a laptop.

I'm not saying this to be rude or short with you, it is just on this truck the control systems are advanced enough that even a seasoned tech can have trouble getting to the bottom of a problem, in the training modules we take they are classified as advanced so training is more involved than normal. And being an intermittent failure this makes it even more difficult to pinpoint the problem.

Good luck