I have been having the EXACT same problem.
I deleted accounts, deleted keychains, deleted pref files. No luck.
I called Apple, and they blame my ISP, SBC/PacBell for the problem. Of course, I call SBC and they immediately blame Apple and OS X. Round and round I go. In the meantime, I enter my frakking password 100 times a day.
If you encounter a solution, please post here, others are having the same problem. Do a search on the boards here. Sadly no solutions yet.