Hi Mick, Sorry for the slight delay. Summer Hols here so a bit of work on for the Forklift repairers. (Shut-down work) Anyway, good to hear from you. I can send you better 'suggestions' if you give me your email. Mine is chevron at pin dot co dot nz

A couple of things to check. Con 3 is the Line Contactor, the one that pulls in when you turn the key. Is that the one that drops out. ? If so, check the following. Firstly, I assume that the circuit is as it should be with Con 3 pulling in on the SPRINGloaded keyswitch position. Con 3 pulls in on 72 volts which is easilly measured at the coil of Con 3. measure with the key over on the spring position. When you let go of the key, the voltage at Con 3 coil drops to something like 22 volts or similar. Make sure you have those two voltages. I cant help but think that you may have a battery cell or two out and that test will show you what you are starting with at the coil. NOW. If you HOLD the key over on the spring position so that you have your 72 volts, keep holding it and then drive the truck or use the hydraulics to make the fault appear. If the fault does NOT appear.. it means that the CON 3 is dropping out through lack of battery voltage OR, that Con 3 coil is graqdually going short-circuit and needs a lot more voltage to hold it in. the 22 or so volts just wont hold it in when there is extra load on the battery (slope or hyd). There is nothing magical or electronic about the keyswitch Con 3 circuit, it is a plain old 'Apply voltage and it should work' circuit.
So, check the battery for all cells working. Check this with a load applied which should best be mast at full height and pull the lift lever and hold for 5 secs or so. Watch the battery 72 volts and see if it drops like a stone. (Expect the keyswitch to drop out of course, that is why we are here.... ) Get somebody to hold the keyswitch over so that you have a full 72 volts at the Con 3 coil while you do that test. As I say, it sounds like a battery voltage problem but the coil could be faulty. A coil needs MORE voltage to hold it in once it gets hot. A coil that would hold in on 20 volts cold will require maybe a minimum of (say) 30 volts to hold it in when it is hot. If yours IS getting hot, expect a short circuit starting to arrive.
Of course, if the circuit is not wired correctly and you dont have that lower 'hold-in' voltage in circuit, then the coil of Con 3 will have been overheating for a while and we are then back to the possible start of a short circuit. the 22 volts (or similar) that Lansing set their 'hold-in' voltage to, ensures that the coil of Con 3 stays cool over a long period. If this has been bypassed in the past, the coil may need changing. Let me know how you go with this. Dont get over complicated as it is just voltage with no PCB's interfering with the operation of the keyswitch. Cheers.