...This could be a problem with FTB monster itself. Everytime this happens i suddenly can't connect to steam. Could be that the error occurring is causing a fault with our net connection...
draze
I was having a similar problem to this at about 2am UK time yesterday morning, but with the Vanilla server. Basically, internet would be working fine (I'm streaming music at the time), then log into Vanilla. About 10 secs after I log in, my streaming music stops ... and a few seconds after that I'm booted through lack of connection.
To add some extra weirdness, I had complete lack of internet access afterwards - but all my network tools and troubleshooters said I was connected. Steam, IE, Chrome and Firefox were showing no connection - I couldn't even connect to my BT home-hub's admin menu's! If I restarted the wireless card (via Disable - Enable device), then I could connect and be back up and running again ... until I logged into vanilla again.
I think the 3rd time I tried to log into Vanilla (after trying a few tests with other online software), it booted me at login, saying the authentication servers were down for maintenance ... but when I checked the MC status page they were working (but got the same error on subsequent login attempts, so gave up!)
Weird!