Well as mentioned above there is a bug filed for this behavior. I guess if you open a browser window, theres no internet? If theres no internet at all, then this is the issue the above bug was filed for so disconnect from network (wifi icon in system tray) and then reconnect.

General concensus is that its probably a maemo wifi driver issue. I rarely get this for files < 350 mb and maybe once per file for files > 700 mb or so. -Possibly-, reducing the max download/upload/peers settings in transmission could avoid the problem, but at the cost of giving up performance.