Jerry,

As long as I don't run the rear window defroster at just above idle and steady state throttle it will maintain 13.8 or so.

Upon application of any throttle the voltage immediately drops but will come back up as the engine maintains the higher (but constant) RPM.

Application of full throttle even with no load (flicking the throttle by hand) can cause the voltage at the battery and alternator to get to just over 12v.

I've measured the voltage at both the battery and alternator and there is at most a couple of tenths of voltage difference.

Maybe that just the way these are, but my 95 automatic one doesn't behave this way - you can floor the throttle all day with no drop in voltage.