weird alternator issue

[clownracer]

my alternator is charging for 15-20 min @ 13.8v then shutting off......then randomly coming back on to charge for another 10 min or so

im pretty sure this is not a loose wire because its happining with the truck just sitting in the driveway

 

[SunlitComet]

faulty internal regulator IF. it is a straight charging or not. nothing like varying voltages when it does charge but wiggle you dash wires underneath and fuse box wires carefully. just in case.