Cat C 15 Acert rebuilt problem

Hi, wonder if any of the Cat gurus can help me, this is what I got,
09 C-15 Acert twin turbo that's deleted.
we overhauled the engine put a new Head, put new wire harness inside and outside of the motor, new fuel lines from the tanks.
The problem we have is the the truck runs perfect, great fuel pressure, great boost, but as soon as we take a hill and need power the RPM starts to fluctuate 300 to 400 RPM and the truck feels like it has no power, the boost and fuel pressure stays normal, once it hits flat country and don't need lots of power everything is normal again.
Is it electrical? or fuel issue?

any insight would be appreciated