Tach needle bouncing

[Sixt8Chrgr]

Hi Guys,

When I have my AC fan on med or high and come to a low idle situation my tach needle will raise up to approximately 4K rpm with the engine idling. Once I pull off and transition into a charging situation from the alternator it registers correct rpm. What could cause this?

Same situation when I have the turn signal on under a low charge situation. The tach needle will fluctuate with the turn signal cadence. When I bring up the rpm it registers normal. What could this be?

Thanks for the help guys