So I recently swapped a 4.6 from a 98 crown vic into a 77 F100. I used a tanks inc in tank set up with a walboro pump. This truck has run a total of maybe 3 hours since finishing the swap. On the second time having it on the road, it got to where the engine would stumble and almost die every time I would give it throttle. Luckily I was close enough to home to idle along the side of the road the rest of the way. The pump was noticeably louder and higher pitched than normal. I took the fuel filter out and inspected (it was fine) and replaced, and the fuel filter returned to normal sound, but wouldn't start. Keying on and off over time had the pump slowly build back up to the high pitched loudness and would keep it at a rough idle.
I know this sounds like the pump is bad but it confuses me why the pump will return to it's normal sound after a line is disconnected (presumably because its letting the pressure out) and then build back up. Being a 98 it has a return style system and the computer can only turn the pump on or off, not run it faster. So I would think the higher pitched sound would have to be from fighting increasing pressure.
Maybe this is obvious and I just don't want to admit I'll have to drop the tank again and replace the pump lol. But if anyone could confirm or give me ideas on it being something different before I do that I'd appreciate it.

 

Is the FPR connected? Can you feel (fuel line) or see fuel returning to the tank?

 

Just went and checked, no fuel was moving through the return. I actually disconnected the return (and with the fuel pump built up to high pitched and loud) and ruan the engine at idle and zero fuel out of the return.
Fuel pressure regulator is all connected. Then while idling I disconnected the vacuum line on the regulator, and felt vacuum on it, kinda fiddled with it, reconnected and fuel started coming out the return. Reconnected the line, idle seems better and rpms can be slowly built up with moderate success. But anything close to a quick jab and just hard stumble.
Now I'm really stumped. I should mention I already replaced the regulator earlier and didn't see any change at that point.