My alternator isn't charging correctly?

Hey guys and gals, about a month ago I replaced
the factory alternator with a remanufactured one
from AutoZone because the old alternator wasn't
charging enough. So the first one was a dud
remanufactured so I replaced it with a tested in
front of my eyes remanufactured alternator, and
that seemed to work for a little bit until last night. I
was driving to work and I got the dreaded code of
"check charging sys" and my stomach sunk
because I've been through 2 alternators. But I
noticed something odd. I only got that message
when I was at a low RPM or around 0-20mph and
the code and battery light would go away after I get
over 20mph and or above 2k RPM. Should I check
the fuse box for #26 fuse or the master fuse. Or
could it be the fusable links? I would really
appreciate some advise.