Charging system is not working, and i dont know why?

Alright, so i bought this car for $500 back in may. i did a lot of maintenance
on it, like replacing the wheel hub and bearings. i also cleaned up the
starter, because it was corroded. It still clicks over sometimes, but always
starts after a second. i also replaced the battery about 3 months ago. A
couple weeks ago i was driving and i noticed my headlights were dim and
my wipers were a little sluggish, so i looked at the voltmeter and realized
that the battery voltage was a little below 12. i continued to drive it for a
couple of days and it would be above 12 sometimes and other times it
wouldnt, so i took it to get it tested, and they told me my battery was fine
and it was a bad alternator. So, i replaced it. it worked perfectly for a couple
of days and then just started charging sometimes and then sometimes not.
So i took the plug off the alternator, and it was severely corroded, so i
replaced it too. and checked the other side of the wire to make sure it
wasn't corroded there too. it wasnt, but it is still charging sometimes and
then not. i have no idea what it is.