HI,
Sounds like a Buffering issue.
Are you also losing Audio ?
In iChat Menu > Preference > Video section try the Bandwidth Limit at 500kbps
This is fast enough for 4 way Video chats but is below most people's Service speeds so it smooths out extreme variances in the Service.
iChat tries to Buffer the incoming Video and Audio.
It will drop individual Frames to try and smooth out the feed.
There does come a point when the Audio will go out of sync (too many Frames dropped) and the Audio will get cut)
If you are going straight to Video disturbances then it is more likely your ISP's internet service that is at fault.
Open the Connection Doctor (Video Menu) during a Chat as this may give more clues
10:28 PM Monday; September 17, 2012
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Lion 10.7.2)
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