The reason you're getting lower readings at higher rpm is that your ignition
system is
drawing down the battery to run the engine
and the juice isn't being replaced.


Assuming your alternator has an external regulator-
What's the cost of a new voltage regulator vs the "minimum diagnostic fee" in
your area? If you can get a voltage regulator for less, do so and put it in
place. That will either solve your problem, or almost certainly prove that it's
the alternator.
Worse case result if it is the alternator is that you now have a spare
regulator (the one you took off but that proved to be working).