0x80042108 Error with Outlook 2003 / SBS 2003 Premium/ ISA Server

I am operating in a Windows SBS 2003 SP1 environment. All workstations are
using Windows XP SP2 Professional and Outlook 2003.

All Systems will periodicaly receive the error 0x80042108 Outlook is unable
to connect to your incoming mail server. This only after having been
successful at completng the tasks of sneding and receiving not 5 minutes
earlier.

I am using ISA Server 2000 in combination with Symantec Client Security 3.0
... So, I have, at one workstation, uninstalled the Symantec Clients for both
AntiVirus & Firewall, with no change.

I have increased the timeout period in the advanced settings in Outlook and
I have even resorted to deleting and recreating the account at the
workstations. Still no changes .... Help, would be most appreacited.

Thanks in advance......

p.s. we changed ISP and POP hosting companies recently to SBC and continued
having the problem despite changing both SMTP and POP servers. ??? !!! ???