I have a 1991 F150 5.0 losing power under load. It's fine when its cold but loses power when it warms up. I've changed fuel filter, plugs, wires and I'm about to change cap and rotar. Unfortunately, my father had passed and it was his truck. Any suggestions would be helpful.

 

Welcome to the site..
No CEL?

If a 'flat throttle' or hesitates at steep throttle angles that then improves quickly on release or returned to more normal load/grade positions,, a good start point is to install a fuel pressure gauge on the fuel rail and monitor the pressure during an event.. Misfires can feel similar to fuel starvation, but given the parts replaced, I'd prove out spec fuel availability.
Restricted cat, ignition, fuel ( delivery and injection) egr etc issues all can cause a loss of power.
If no CEL, I'd make sure fuel isn't an issue first off..