Electrical Issue

A few weeks ago I had a battery go out. I had it tested a few days before so through tests I thought the alternator was bad. I swapped alternators and still couldn't get battery to take a charge. Took old alternator and battery in to be tested and found that just the battery was bad. I left the new alternator in since I wasted the time changing it. Yesterday I was driving on the highway and the my electronics started to go out the same way my last battery did. I pulled over to wait for a ride and while waiting the car never died like before (was only 10 degrees out so I wanted some heat). Checked the battery while running and was steady at 8.5 volts but the car continued to run. I took out the battery and put it on a charger for the day and it tested out fine. I went back later at night to put the car on a trailer and get it home. The car started just fine to get it on and off the trailer. After taking it off the trailer I let it run for a few minutes and checked the battery volts while running. It was steady at 14.0v. It appears that my alternator was charging the battery now. Does anyone have any thoughts on what could be going on now? It's a 2002 Jetta TDI 1.9l. I'm not very good with electrical so extra detail will help me understand your thoughts. THanks

Did the alternator light come on when the issue happened? The alternator clutch may have acted up, they can sometimes stop clutching properly and then work later on.

Also, there is a 'exciter' wire that goes to the brushes/regulator on the back of the alternator, make sure its in good standing, if this wire loses signal the alternator wont charge, and you wouldn't even get a light. This wire runs along the front of the engine, past the starter area and can be a area prone to corrosiveness, there is two wires, one should be power, the other ground.