Need Help w/ Hitachi EX-120 Engine Problem

I have a 1995 EX 120-3 with about 8000 hrs. Recently when it gets warmed up it starts to bog down & then start missing under a load. The more I use it the worse it gets until it misses to the point of quitting. It will run ok at an idle or max rpm without a load. If I leave it off for 10-15 minutes, it will work ok until hot (10min) where it starts again. I've replaced all the filters, checked for H20 in fuel, & check lines. The fuel pump seems to pump ok but I haven't put a pressure gauge on it yet. No visible leaks and I don't think it is getting air in the lines but I haven't cracked the bleed line when it's missing yet. Clean air filter. Good vent on fuel tank. The throttle linkage stays at the max setting while it begins to bog down. I haven't owned the machine long enough to know it's history or care.

Any ideas? Is there anything electric that affects the fuel delivery through the pump when a load is placed on the system? I'd appreciate help or a trouble shoot process to find the problem. Is this typical of a wore out injection pump? Obviously the most expensive fix.

Thanks - Bill

Need Help w/ Hitachi EX-120 Engine Problem