have a "75 Volvo DL it wants to stall?

Question

i just put a tune-up, bought a new fuel pump (external) it will run for a few hours, then can hear a loud humming noise from the tank and it will stall,,,,????

Answer 1

Check/ replace the gas cap. Or have it checked. The breather hole in the cap might be plugged up, or nearly so. I'm not familiar with those caps, but some have a different type of breather mechanism, rather than a little hole.
On lawn mower gas caps, Mud Wasps will lay an egg in those holes, and the mower engine will starve for gas and quit. In order for fuel to come out of the tank, there must be a way to get air into it, or a vacuum will form and nothing will flow! Even with a fuel pump.
Try this--next time it does that, slowly take off the cap, and listen for a slight hissing noise, which will mean the breather is plugged. The humming noise is the pump drawing a vacuum in the tank, and vibrating the tank!