I'm trying to narrow down the regulator, alternator itself, and wire connections. I'm renting a room and don't have any place to actually "work" on the car other then checking voltage.
The problem has recently become much worse and I have a trip across 3 states coming up in 12 days!

PREVIOUS ISSUE: I had a shop fix an issue with my low-beams not working ~ which they fixed 2 months ago (they spent hours looking) ~ saying it was a loose connection under the dash. Driving home, this issue started intermittently. I brought it back the next day but they couldn't replicate the problem, saying the charging system is a little week and to have it tested again in December.

LAST MONTH: After about an hour of driving, my voltage drops to 12.1 volts (my installed voltage meter, replacing the cigarette lighter). Adv. Temps Outside were 75*F-ish.
LAST WEEK: After 30 mins of driving, voltage drops to 12.1v. Adv. Temps Outside were 70*F~ish.
YESTERDAY: After 10 mins of driving, voltage drops to 12.1v. Adv. Temps Outside are 50*F~60ish.
__TODAY__: After 15 mins of driving, while waiting at a light, voltage dropped to a steady 12.1v. Turned around in a parking lot, voltage went back up to 14v. For the next 10 mins, it fluctuated between 14v and 13.4v. When parking, voltage jumped up to 13.8v. In park, raising RPMs up to 1500 gives a steady voltage of 13.9v lowering to 13.7v after about 3 mins. Laying off the gas to idle it fluctuated between 13.7 & 13.8 sporadically. Temps outside were around 55*F~ish.

... In all cases ...

1. Speed limit is typically 40mph.
2. Alternator charges sporadically, all around 13.6v to 14.1v.
3. Voltage will not stay perfectly steady while moving, fluctuating around 0.1 or 0.2 +/- every second or two.
4. When stationary, I shift the vehicle from drive into any other gear and voltage returns to 14.2 until I start actually driving more then 3mph.
5. More so early on: Occasionally, while driving, voltage will randomly switch between 12ish to 14ish for a few seconds ~ i think this might be related to road conditions (down slope usually returns but not always).
6. Breaking appears not to affect this issue in any way.
7. Starting the car is normal, voltage is 14ish.
8. I have no check-engine light or battery light.

My Thoughts:

1. I'm thinking it is not the regulator because my voltage does not exceed 14.2. Could this be incorrect? I read somewhere that it could fail in one direction (+/-) & not the other.
2. I originally suspected a loose alternator cable but doubt myself because shifting provides consistent results.
3. The shifting part really confuses me!
4. Recently I thought the belt might be slipping but I hear no sequel. Perhaps the pulley is seizing on the alternator itself? (..I can't see it while in drive.)

I have a BlueDriver module (sensor reader & code-scanner via odb2 port) but I couldn't distinguish any patterns. Only voltage available was the O2 sensors.

2002 Honda Civic Ex.
Any thoughts would be most appreciated. Thank you.

 

Check for a failing alternator. Make sure the bolts holding the alternator and its bracket are tight as these are the ground points for it.

 

The shop I originally brought it back to said it was fine, just a bit under powered and ought to be checked in a few months. I've read of load-testing failing and I think this is a similar circumstance. When the car is stationary, in a stationary gear, my voltage seems fine.

The loose bolts seem to be a possibility ~ that would just take the cake, wouldn't it?
I'm taking it into a shop this Tuesday to have it checked & belts replaced.