Windows 7 - Outlook 2007 SMTPauth fail/fix/fail/fix ?? why

gang I am embarrassingly stumped on this.
Lawyer moved to new offices recently. ISP stayed the same and level of service the same. The connections, and the cable modem [which talks to lawoffice router] is new.
NO changes were made to the Outlook email setup on the system in question.

about one month on, the email account failed to get authorized. after a long story, I finally just duplicated the account, and all was well until it happened a second time. This time I tried disabling the AVG mail scanner, but still could not get credentialed.
Repeated the setup of a new account with exactly same settings again, and uninstalled AVG, installed AVAST. All was good for about a month, now it has occurred again with one diff: the POP3 authorization is good, the SMTPauth is blocked.

ANYone have a clue why this is occurring? the ISP is Centurlink, the address is @centurytel.net. the other users in the office have no issues.

and just in case it is related: the same system had several instances of TCPIP overrun: that is, the security feature that stops connection attempts after a preset limit is hit kicked in on two separate occasions, not in sync with the mail issues although close.
I checked for malware and it is clean, according to ordinary measures. I did discover that the preceding tech had setup the networks in a bit of a mess, with one system on one workgroup and other resources on another, but I have to say that prior to the move no big issues were noted with that arrangement. at any rate I put them all in the same homegroup.

I'm clueless as to why/what is happening the seems to "mark" the outlook mail account as "bad" or inserts something in the string that causes authorization to fail. I repeat all the same settings afresh on a new account and it works... for about a month... what IS going on?

thx
z