Dellman,
I am guessing you have a SI, or internally regulated alternator on your truck. If it was tested professionally, the results would only be valid if the it had a integral regulator, hence the assumption.
Behavior as erratic as you indicate would probably show up during a comprehensive test, so my guess is that it would be a wiring or connection issue.
If the alternator (three wire) can't 'see' the voltage in the system, it cannot regulate it. It monitors system voltage with the little wire in the DA plug marked #2.
I honestly didn't think, even under full field output, that a stock alternator could pump out 17 volts. It sounds like it can.
Hooking your meter to the battery would be a good test location. Get a 'before' voltage reading and a running voltage reading. These should be in the area of 12.5 and 13.5 - 14.0 volts.
In my estimation, a professional alternator test doesnt include the phrase Auto Zone, Kragen or Pep Boys.
If your alternator uses a external regulator, then giving the alternator a clean bill of health is only 1/2 of the troubleshoot.
Stuart
