1990 Ford Bronco charging problem

To any ford experts:

I own a 1990 Ford Bronco XLT with a 5.8L V8, Well I'm having a charging issue with my truck and I'm at a loss on whats wrong or why its doing it. Let me explain whats going on. I have a new battery and my alternator is pretty new but have had it tested twice and I get the same answer that its putting out 14.5 to 15 volts. Well when its cold I can start it up and my volt meter reads that I'm at full charge, after I drive for awhile it starts to show me less or slowly drops to 12 or so volts and it dances back up like it just caught a charge and stays for a bit and slowly drops to a low to where my battery is the only thing starting my truck. I realize that my pigtail connector to my alternator was close to my block and melted the wires a bit so I pulled them apart, check the wires and black taped them up and re positioned the harness away from the head on the motor. Yet I get the same thing happening. Here's what I've done in the last few months to my bronco. I've replaced the fuel tank (have a few leaks), replaced the ignition actuator (it broke 7 months ago so I was starting it with a screw driver on the starter solenoid on the right side of the engine compartment), replaced the battery and cables. Now is it possible to run my negative cables from the battery to the alternator bracket then to the frame. Just wondering. Or may be clean my connections. At a loss here, just don't know where to begin to fix the problem.