The Xantrex XAR regulator uses a single
red wire to sense the battery voltage, draw the power to operate the regulator, and to supply up to 5A to the blue alternator field wire. Any voltage drop in the red wire is a bad thing. I have a feeling that the cycling caused by a bad contact has steps something like this:
1. The regulator starts up with no field current being provided to the alternator. The current flow in the red wire is low (just enough to power the regulator electronics), thus the voltage drop in the red wire is also low. The regulator has enough voltage to supply its electronics
and begins to charge the battery.
2. As increasing amounts of current flow to the regulator, in thru the red wire and out to the alternator in the blue wire, the alternator output rises. At the same time the resistance in the red wire's bad contact causes the voltage sensed at the regulator to fall as the red wire current increases. The regulator reacts by increasing the current to the alternator field in an attempt to increase the battery charging rate.
3. At some point the voltage at the regulator falls to the point that the electronics
in the regulator shut down. The current to the alternator stops, and the alternator output stops.
4. With little current flowing in the red wire the voltage at the regulator rises, the regulator restarts, and it all begins again...
I have never measured all this while it was happening, but it is my guess.