Careful with the upgraded alternator business. Most of them spit out nearly the same current supply at idle as the stock alternator. The max rating is measured at high rpm under heavy load, and then the alternator is advertised that way: "200 amps of output!" Yeah ... at 5000rpm on a test machine that's loading it down with 200 amps. Do you have a lot of aftermarket stuff that would be drawing more than 55A (the supply of the stock alternator) while running down the highway? I have aftermarket lights on the front of my truck that can draw 30 amps when they're on. I run a 65A alternator and it's fine as long as I'm above 1000rpm. A 200 amp alternator probably wouldn't have much a difference below 1000rpm so I haven't "upgraded".
Here's an example of alternator curves for a different vehicle - it's still representative of the concept. Look at the lower rpm range. The "smallest" alternator is actually able to supply more current at 2000rpm than the "bigger" ones. Again, not the greatest graph, but it was the quickest one I found googling.
It looks like you've either got a regulator getting ready to fail or corroded wires/dirty ground points. Cleaning up your grounds is free, even if some of them are difficult to find. Finding corroded wires takes time and a multimeter. You can get a regulator from almost any parts store, though they'll probably have to order it. Find a NOS unit if you can, I've had a couple parts store regulators go south in less than a year. Either way it's best to buy and carry a spare - they're easy to swap out. Two screws and 5 minutes. There is a chance your battery isn't holding a charge anymore too, but I'd go there last.
And after that, there's the whole world of electrical system upgrades. Larger ground wires certainly help a lot. Replacing wires that are very corroded is not only helpful, but necessary to prevent potential fires. I'm not trying to be a shameless salesman here, but I make my Fusible Link Replacement Kit with larger positive cabling from the alternator to the battery which helps the alt charge the battery more effectively. Most people see an improvement of 1 volt at idle just with that. But electric stuff needs to be thought of as a complete system. Some little improvements will help a little. Doing many improvements has a bigger cumulative effect, greater than the sum of its parts. Right now, with all the improvements I've done, I can sit at idle with the headlights, windshield wipers, both HVAC fans, blinkers, and radio going and I'm at about 13.4-13.7V.
