Jumpy ammeter

[GTS4401969]

I have a 70 swinger with rally gauges and when it starts the ammeter show a charging which is normal and then after a minute or so it starts jumping back and forth across the whole sweep real fast.... help what is causing this? dont know if it makes a difference to diagnose but it has a 340/4 speed

 

[cudadriver]

I'm guessing a bad regulator.

 

[nothingbutdarts]

The 70 could have the old mechanical regulator which has points in it versus the newer electronic regulators used I believe in 71 up that usually will charge steady.

The points get old and carboned up. I could also however be a bad diode in the alternator which will make it not charge as good as it would when it's working correctly. With this problem the ammeter will also jump around somewhat.