A Jabber login happens on either port 5223 or port 5222 depending if SSL is being done or not.
Googletalk uses port 5223 as it normally use SSL to connect.
If the server is dropped (or drops you) then iChat or Messages needs that connection reset.
In most cases this can be done by unticking the "Enable the Account" box in the Account info tab (App Menu > Preferences > Accounts).
I have found you can change the port to the Google (talk) server to 443 and it will be accepted.
you have to change this whilst logged out.
I find this does not seem to drop the connection as frequently as the regular port.
For other Jabber servers it can be more difficult to find a port that works.
Most do not allow a Non SSL login (Port 5222).
It can be trial and error to see if they will accept a port such as 443 in place of port 5223.
(443 is used for Secure site Web Browser logins like Banks and the Login process here at Apple Support Communities and also by some mail servers)
9:29 PM Friday; November 30, 2012
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously