Bit off topic , can't keep one of my forklifts (Moffett) running

We carry along to job sites the fork truck lift you see hanging off the back of trailers, its great tool but, its a PITA too .. I have one that refuses to stay running, we have had it in the shop mutiple times both locally and on the road , its powered by a Kubota Diesel engine .. it acts like it runs out of fuel, you let it sit for a bit , something 10 minutes , some times couple hours , it will fire right up and run fine again

replaced fuel pump at least 3 times and always seems like is is fixed for a day or so, then .. right back to dead


any ideas ? My lift mechanic, my inhouse mechanic ,, no one can figure it out