Sounds like the UPnP on your modem/router is unreliable. Some devices do not have reliable implementations of UPnP. UPnP allows a program to send a request to the router to open a port for just that session.

Port forwarding a device is more reliable than UPnP .. of course depending on brand & model.

Also something else that can affect reliable connection is too many incompletes as it can flood connection ... ie: give the router problems .. this will especially apply to UPnP mode but not only. -> Tip: Purge the Downloads Queue! (click on blue link)