For some mysterious reason, messages go from my inbox to my spam folder. I mean, the messages will be in my inbox, stay there for a few days, and then I can't find them anymore bc they are in my spam folder.

This only happens occasionally, but enough to lose some important stuff. I don't have any filters.

Gmail support gives me the runaround. They have no clue.
Anyone else experience this? Anyone know what the problem is or how to fix it??

I don't know exactly why this happens, but try sending all your inbox mails to the archive, using the Archive button so you don't have all your stuff on the inbox (you can read archived mails whenever you want by going to Archive link in the left).
(I don't know if it will solve the problem)

Also, try adding frecuent people who send you mails to your contact list, so GMail identifies they are "friends" and don't spam it.

For those mails who go to spam, don't move them back to the inbox, use the "This is not spam" button so GMail learns that is not spam.