Komatsu D37E loses RPM when hot

I have a Komatsu D37E 1989 that runs excellent except after about thirty minutes to an hour of pushing will start to bog and lose RPM. It wont stall as long as you quit pushing. If it cools down even for a few minutes it will be fine for a while and then do the same thing. Acts like it is starving for fuel. I have changed the filter and bled the system, no help...

Any idea or directions on what to try next.

Thanks

Komatsu D37E loses RPM when hot