I am not sure, but my guess is that your router is closing some of the ports that your computer needs to connect to the outside. Sounds like a firewall issue.
Although, looking at the info on the net, it seems that DropBox uses the same ports as your web browser. So, if your browser works well, DropBox should too.
Comes down to the settings in your wireless box. Or maybe the strenght of the signal? If your computer is too far from your wireless router, the info takes too long to get there and you computer quits looking for it.
