So I wonder if when it's cold, when at home and at the store being tested, that there is an internal connection that is good but once heated up, this connection goes south ?.
Yeah I tend to agree with u.... as in my post above I did say assuming all the joints are good ....
yep possible crimps on feild coils..
but more likely/ common to be a dry solder joint on the internal regulator diodes.... they warm (get quite hot actually hence the heat sink and fan on the front) up, crack in dry solder joint opens.. in effect a diode drops out...voltage drop.. just as if a diode dies fault.
If it is, it is very likely that the diode being effected will die in a yr or so time...
This dry joint thing is same fault as in a starter.. the header heat sink myth... start works fine.. go to the shops for milk, starter clicks... big current, causes dry joints on the starter armature....cools down and fine. The heat is due to the very high initial currents going thru... not heat from the headers... dry joint caused by initial being too far advanced and the initial current draw extremely high, over time dry joint.
So get them to swap out the internal regular diode assembly....
Would lay odds that will fix it
Oh to test a starter armature . one puts the armature in a oven and heats up till just too hot to hold.. then drop in a growler or check continuity with ohm meter
Checking continuity with regulator diodes doest work because they are set up as a bridge.. to do so the other diodes need to be unsoldered 1st.. sort of defeats things lol
So its just a matter of going around and re melting all the joints.