'81 flareside 4x4 project

I replaced the fuel pump about a year ago shortly after I bought the truck.  Probably would be a good idea to pull the line off the carburetor to verify flow.  

I think it is probably ok though, I have a clear filter on the line, which is nearly empty when the truck acts up.  This morning it cranked over and started easy, and the filter is full.

When the truck is running poor, if I pull the line from the carb inlet there is a lot of pressure and gas vapor.  It actually is pretty dangerous as it spews this mix all over the hot engine bay.

Man, that's strange to me.

How's the tank vent? Have you tried loosening the gas cap or taking it off when it's doing it?

Have you replaced the rubber hoses?  On Dad’s truck they were so porous that they allowed air in, killing the pump’s vacuum. It would run ok for a few miles and then stall.

There’s a hose from the tank to the steel line on the frame, and one from the steel line to the pump.

nmchuck wrote

When the truck is running poor, if I pull the line from the carb inlet there is a lot of pressure and gas vapor.  It actually is pretty dangerous as it spews this mix all over the hot engine bay.

This makes it sound like a carburetor bowl vent problem. If you have pressure in the carb, you can't pump any fuel into it!