i was thinking about this after i made my last post and the control handle did come to mind, i know they had issues with these at one time but since then i believe they did re-vamp the handle to a new style so i doubt that is it.

As for the tech you know that is an independent? unless he had worked for yale in the past i doubt he has the software to interface with the control system on this truck.

According to the manual i have on this unit it has a curtis control system , a 1310 master control unit and 2 1243 motor control units and possibly an optional 1254 pump controller. The premium trucks had this pump controller and generally came equipt with a premium display. Since your display doesn't seem to have any special display properties i'm suspecting it is just contactor hydraulics? The way to distinguish this easily is when you activate the hydraulic function the motor just goes wide open to a preset speed, no scr variation, its either on or off and you hear a contactor engage when you move the handle. That is something no effecting the truck so we won't get into that.

When you have the top panel off i'm sure you noticed 2 motor controllers and the master controller in there, make sure these connectors are clean as well. And also there should be one or 2 contactors for 'traction or line' and if it has contactor hydraulics there would be one for that. Check these contact tips and make sure they are not carboned up or burnt/pitted out. The 'line' contactor if carboned up can cause alot of undue issues due to bad connection from carbon or burnt tips.
These controllers also have an LED light on them, when they are in normal run mode the light will be a steady blink , on... off...on...off, but if there are any issues with the system they will blink a code much like a car or forklift does that has a MIL light, it should flash the code in sequence in digits, like if it had a code 43 it would flash 4 times, then pause for a second and then flash 3 times. It will repeat this code as long as the fault is present.

Now as far as the drive system, like i said, there are a couple ways you can get into that, 1- through the dash display which is rather cumbersome and not my first choice. 2nd - the laptop which would be my first choice and thirdly - curtis makes a handset which can plug directly into the controllers. I have used both the laptop software and the handset and now i believe what you probably need to do is go into the main controller and reset the settings of the truck back to default. There is a setting in there that is probably out of range and causing the truck to not run properly. Since you are not getting any errors on the display this is my only thoughts on why it keeps kicking out.
Not long ago i had a yale order picker doing almost the exact same thing, but it was throwing errors and someone had gone in and made adjustments and inadvertantly changed a setting they shouldn't have causing the system to have a conflict causing the truck to quit running and it was intermittant just like you are experiencing.
If you can get ahold of either of these devices we may be able to get some things straightened out without much more undue pain and suffering (so to speak) ;o)
Curtis makes 2 ways to gain access to thier motor controllers directly...

1 - the 1313 handheld programmer (last time i checked these run around $600 and they come with different access levels, you want one with at least a service or dealer programmer level, the user access level will not gain you access to the menu systems you would need to make all the necessary changes you might need)

2 - 1314 PC Programming Station (not sure of the pricing on this but i suspect it would be around the same price. ( it comes with the same access levels so you still would need a high level access not a user access level.

If you want to learn more about these go to curtisinstruments.com and look in the products page.

Of course you still would need the service manual to make sure you get the controllers set the way Yale designed them to run with this truck to insure correct operation.

Usually when i would have these kind of issues with these units after eliminating the battery and/or connection issues i would always go into the controllers and make sure all the settings were correct and see if anyone had been in there futting around with the settings trying to make it run faster or something like that. In most cases i always found someone had and i would have to reset the system back to default settings. Once i did this the unit generally came back to life the way it should.