Essentially iChat or Messages will believe it does not have enough Bandwidth (your computer's portion of the Internet speed).
As you say this is stored in com.apple.ichat.plist each time you launch the app.
Open any Finder window
Use the Go Menu whilst holding down the ALT key (Lion Upwards)
This will make the Library appear in the menu list and you can select it.
Navigate to Preferences.
Find com.apple.ichat.plist and Drag it to the Trash.
10:17 pm Friday; January 3, 2014
iMac 2.5Ghz 5i 2011 (Mavericks 10.9)
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