Hi Sean, Sounds a bit like a cell failure. Any 'smelly' cells or fuming.? The voltage you give is high unless it has just come off charge and if you have a dodgy cell or cells, that voltage would drop alarmingly if you measured it whilst stalling the lift or even just raising the mast. (if you can). You can voltage test each individual cell and expect around the 2.05 volts or even a tad higher if it is a fairly new battery. Best voltage per cell would be 2.35 or thereabouts on a newish battery. If you have a cell that is extremely low, (1.5 or lower) test it again while stalling (say) the tilt and see if the voltage drops away rapidly... If it does, then that cell may have failed... It can be shorted out to restore full operation. You can short out maybe three cells before you have to replace them. More than 3 cells will upset the Charger and it may not turn off overnight... (A modern Charger would not even bother to turn on)

Other reasons for your fault.? Maybe check the line contactor. That is the one under the floor, right hand side and operates when the key is turned on. Look for stiff operation, (try by hand, it should be free). The NYlon Pivot rod for the moving contact arm gets stiff when warm. It can swell up and stick in the ally mounting bracket. When the moving arm drops out or is switched off at the key, it can stick in the not fully back position. When it IS fully back, it has to be in contact with the small rectangular contact block before the key will switch on again. Once it is cooler it may very well end up fully back against the rectangular Contact and the truck will work again.
Look also at the 'L' shaped moving contact on the line -contactor to make sure it is not loose with a burnt bolt holding it.
Sounds to me though that you have a voltage loss problem. If you can get it going, can lift the mast right to the top and then measure the voltage WHILST you are holding the lift lever fully back (Stalling the motor), watch the voltage and see if it drops away below about 65 volts.... (appx) if it does you are going to have to test each cell like I described above. Good luck and let me know how you go. I can offer more info with more info IF you see what I mean.....
If the keyswitch circuit has not been altered, you have to hold the key fully clockwise in its second position to get the Line - contactor in. When you let go of the key, it should spring back to its first position. If yours is still like that then try holding the key over to the second position WHEN the truck is playing up. This feeds the full battery voltage to the line-contactor and it will usually stay in all the time you are holding the key fully over.. (When the key is let go, there is only an 18 -20 volt supply to the line-Contactor which will HOLD it in once it is in. A weak or faulty battery reduces that 18 volt holding voltage to too low and the Line- Contactor will drop out. Good luck and dont forget to ask if you are still having problems.... Regards, Baz C.