Back when I was a mechanic, I had a fire truck with a mechanical 8.3 do the same thing you describe. The governor in the pump broke. Had to remove the pump and send out for a rebuild. It's not a bad job if you have access. It's been a while, but if memory serves me, it's was over $1000 to rebuild.
2002 Holiday Rambler Imperial 38PKD
370 Cummins ISL