Alternator tests fine but not charging

Ok i have a 99 dodge, about 3 weeks ago my
volts dropped to 10 while driving, i assumed
alternator, so i took it off had it tested it
passed, put it back on it fired right up no
problems thought i was going crazy, then it
pulled its stunt again a week later, pulled of
alternator and battery again had them
tested, they passed put them back on, truck
started working again, running 14 like its
supposed to, then it pulled a stunt again last
night, its like the alternator is choosing when
to not work and start working and usually
works after i jossel it aroumd taking it off
and putting it back on, any ideas?