[shorod]

Sounds like you might have some corrosion in the circuit somewhere. As soon as you load the circuit, the draw causes the voltage to drop. Are you using a multimeter or a test light to check for voltage? A multimeter has high input impedance meaning it barely loads the circuit. This is good for most measurements, but makes it difficult to find circuits with little load carrying capabilities.

You could try a test light and see if the DG/Y wire is able to brightly light the test light. If it does, then you should check the feed to the fuel pump for low resistance to ground. If it were currently a dead short to ground I'd expect it to again blow the fuse. If it's just a low resistance to ground, it may take it awhile to heat the circuit enough to blow the fuse, but still load the circuit enough to cause the voltage to appear to drop.

While none of these measurements will tell you where the problem is, it will at least help you determine where to start looking.

-Rod