7775 john deer skid steer problem

I've got the above mentioned skid steer. 1998, good condition, 1900 hours. Problem I'm having is after about an hour of running now, it starts running sluggish, and eventually quits. It acts like its getting air in the injectors. After sitting for awhile, and having the key on with the self bleeding system, it will eventually start and run fine for awhile. I've checked the fuel vent cap, replaced the electric pump, and replaced all the lines to the filter, and the filter. Also checked the pickup tube in the fuel tank. Injectors? Any advice would be greatly appreciated.