It happens to both my girlfriend's and my computers. She's on a pre-Intel iMac, I'm on a MBP. At first I thought it was due to the wireless network, but the problem persists on any sort of encryption, or even if I'm plugged into the modem directly. I'm surmising that we can just chalk it up to crappy coding on the part of the junior high school student or two Yahoo apparently has in charge of their Mac department. Adium, on the other hand, doesn't drop me at all, so I switched over to that.