Hi,
I would test some other situations ton see if you can "prove" it is somewhat linked to Mail or at least something the OS is doing regularly.
I would try different time settings in Mail
I would try iChat without Mail being Open.
The first may highlight it is the Check that Mail does that "upsets" the Internet.
The second may exclude the Mail App and highlight that it is something the OS is doing separate from that.
If you manage to exclude Mail as the cause then look in System Preferences > Date and Time and check if it is using a Time Server ? (in the Date and Time Tab/Button).
Does this fit with the Time Zone you are in ?
If they appear to be OK check the Date and Time settings in your Modem or router.
9:31 PM Thursday; March 22, 2012
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Lion 10.7.3)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously