Lights flicker and gauge jumps around???

How do I check for open diodes in the rectifier? I'm sure I need to take the alternator apart but will it show when tested? I'm having the same issue. I have a new Ron Francis Express fuse box/wiring harness, 1 gauge battery cables, 6 gauge ground from battery to body in 2 places. The fuse box, Ron Francis voltage regulator, are all grounded. Alternator reads 14V and above at idle. At higher rpm it reads 15V. I have checked voltage with a fluke multimeter and have very minuscule voltage drop. For example, at 2500 rpm, I get 15V at the battery terminals and 14.91V using the master cylinder, alternator body or intake manifold as a ground. I'm thinking alternator even though it's new.