[jhv48]

I thought about the gauge going bad, but the fact that when the gauge goes positive again, it goes higher than normal and then slowly falls back to normal charging levels is what I would expect if the engine was running off only the battery for a short time, thus causing the alternator to put more of a charge into the battery to make up for the discharge while it was powering the engine.
If the gauge was bad, I would expect it to return to the normal charging level
once the gauge started working again. Also, when I turn on the fan with the engine running during normal operating conditions, the gauge will make a big jump to the minus side and then quickly return to the zero position as the alternator delivers more charge to compensate for the higher amp drain. When the gauge is showing zero charge at running speeds and I turn on the fans, it immediately drops to about half the negative side and stays there. That indicates to me that the fans and the engine are both running off of the battery, and the alternator hasn't kicked on to counteract the discharge.