Mail 10.6.8 HELP ??

Occasionally, for no apparent reason, my 10.6.8 Mail renders various and random mailboxes unavailable. Various servers randomly reject passwords, although they ARE correct. I confirm and/or reset in Prefs and/or Keychainto no avail. Several days later, again for no apparent reason, the boxes begin to come online again.

For several days now most boxes come online, then in a few minutes ... ALL go offline. imap / smpt / mail / aol / yahoo / pop boxes, everything!

Help?