If this happens after 15 mins then your modem/router device's UPnP may be unreliable. In that case, manual port forwarding may be necessary.
As Sphinx suggested, you may also need to configure your computer's software firewall if you haven't done so already. Instructions for this & the above point can be found in the 1st Sticky in the Connection Problems section of the forum.

Another reason it may happen is if LW is over-worked. If you open LW & resume all of your incomplete files by using the Resume/Find Sources function, then this may make LW struggle if you have many incomplete files. This will queue your files for much, much longer. A queued file means LW is too busy to handle it. And on some people's systems, resuming all files at once can not only make LW struggle by using up all its resources, but may also use up your computer's resources & may affect the communication between your computer system & your connection device. If using the Resume/Find Sources function, apply it to only 1-3 files at one time.