It's beginning to sound like the alternator is not charging much, or there may be too much load on the bike.
The "too much load" scenario: another SOHC4-er here recently discovered his PO had installed a 90/100 watt headlight. No matter what you do with that, it will not charge the battery. Headlights should be 50/55 watts max. or an HID type.
Th e"alternator not charging" gets my vote, because of your description: when revving the bike, the ignition coil currents drop by about 1 amp, so the battery voltage would go up in response if the system were not charging. This rise would be just about .3v to .5v, like you're seeing.
So, look into this part: make or get a jumper to go between the BLACK and WHITE wires on the voltage regulator, with all the other wires still in place. This will jumper the regulator to HIGH CHARGE mode. Start the bike, then check the voltage at idle and around 2500 RPM. It should rise at least 1 volt or more as you rev it up, indicating charging is happening. If it does not, the connections to the field coil or the alternator itself may be bad, dirty, or cooked from some distant past episode. A less-often seen scenario: the field coil could be cooked, or open. This coil should measure about 7 ohms between the above-mentioned WHITE wire and ground (without the whie wire connected to anything: unplug from the voltage regulator first before testing).
Often, the big white connector to the engine has corroded connector blades inside, just from the years. If they get dirty enough, they also get hot, then they can cook. So, check that big connector's blades before possibly replacing any alternator parts. It's pretty rare that alternators or field coils on these bikes fail, but it occasionally does happen. A good one from a junkyard will cost around $30 (out here in CO).
Also once in a great while, the voltage regualtor, which is electro-mechanical, can get bad points inside or an open resistor on the back. I have some old ones that I have "refurbished" by cleaning the contacts, readjusting them, and alighning their performance on a variable power supply, for those days. If you decide you need a good one, I'll sell you one as a "rebuilt", swapping your old one in the process so I can have another to rebuild. Drop me a PM if you get this far.
