Alternator/ Guage

I have posted before but it fixes itself before I can put a mind to ot.
When I start my 03 peterbilt 379 ,old werner truck.
C15.. 13 speed.
I start ,it turns over and all guages come to life.
The batt lite and buzzer cones on and I rev truck up over a 1000 rpm and guage reads 13+ and buzzer go's out.
Turn all my lights on at night and batt light blinks on and off no buzzer.
I recirculat ignition and all peachy again.
Go to use my micro to heat water or somthing and light and buzzer go's off.
I turn off micro or turn off lights and light and buzzer go's off.
Then no problems the next 30 40 hours.
I don't get it.
I think I can fix the issue but need some direction to start.

And help would be appreciated for sure.
Thanks.