I have the 350 crate motor, and new fuel line 3/8, new fuel tank w/ a new sending unit. i installed a holley blue electric fuel pump and it failed so I replaced it w/ a mechanical. thing is with both pumps the fuel pressure guage falls to zero when the engine heats up. When the motor is first fired up its at 7 lbs pressure. ??????????/ Im lost........ this is the second guage by the way. Whats going on? plenty of fuel in the fuelbowl and can see it pumping with the site plug out.