Kenworth T660 overvoltage only on startup

Have the STRANGEST problem:
When I start my truck in the morning the volts go to 15.3 to 15.5 , voltage light comes on, then return to 14.8 after I start driving, light goes out and usually doesn't come back on unless I stop at a red light. Sometimes I will see a voltage fluctuation: up then back down.
Battery load tested all batteries, all in green.
Replaced 3 copper lugs that had some corrosion.
All connections to alternator are tight. Pos and neg battery wires were recently cleaned and sprayed with battery terminal corrosion inhibitor.

I read some where I should check for gunk that might have gotten between cables going to the starter.
Checked alternator with truck running: shows its putting out 14.5 volts.

What else can I look at?

Thanks.

 

14.5 coming from the alternator posts

 

Thanks for all the kind responses. I took the alternator off, took off the rear cover and found an armature that goes to the voltage regulator had broken. It was nearly broken completely off but touching intermittently, hence the voltage fluctuations.
Installed new alternator and back to 14.8 V

Back on the road...