My music keeps disappearing from iTunes (Sometimes)

Sometimes when I open my iTunes up all of my songs will be gone. The songs don't get deleted from my computer though. I can go back to my music folder where all my music is in 3 separate folders and put it all back in iTunes. Everything plays fine when I load all the files back fine but then it might be a day later or two weeks later and I go to open iTunes and everything is gone again. It is just real annoying because I have alot of music with lot of duplicates and songs with no title or something. So everytime I get half way through trying to organize my library, I have to start all over again because the songs disappear and I have to upload them from the original folders again. Anyone have any idea on why this is happening and how to fix it??? and I've tried reinstalling iTunes and am still having this problem.