Reoccurring e-mail problem

[ga.rhythm]

I am running .706, but I think I've had this problem on other OS versions. I have three e-mail accounts, 2 yahoo & 1 gmail, all of them 7-12 years. My gmail account & primary yahoo account are not affected. Every few weeks I am notified by BB that my secondary yahoo that my messages to this account are not being delivered (nor can I send a message). I do not have this problem when I use a PC. I am instructed to go back to set-up & validate my password. Validation is not accepted. I have to go back to my yahoo mail, get a new password, I use it to validate my account on BB, then I change it back to my old password & it works for a few weeks, then this problem comes back up. The only variable I can identify is that I don't send many messages on account, just receive. Is this my problem? Any help or suggestions would be appreciated.

Thanks.