1968 Ford Mustang Gas guage reading "E"

I have a 1968 Mustang where the gauge will fluctuate around the empty point. I have replaced the original gauge (that did not work either) with a Phantom after market gauge, new tank sender and gauge regulator.
• I ran a separate wire from the sender to the gauge to make sure that was not the issue.
• When I put the volt meter on the power side of the gauge regulator, it is a steady 12 volts. On the output side, it initially delivers a steady 5 volts for a few seconds but starts to fluctuate between 0 & 5 volts with the majority of the time on 0. The gauge does work initially when the volts are steady at 5 volts. I put the old gauge regulator back on, it worked for approximately 20-30 seconds than started doing the same thing.
What is the suggested testing process to isolate the issue