About once or twice a day my 2860 running firmware 3.7.4.3 stops any transmission down network cable connections but the wifi works fine.

The only way to restore the cable connection is a cold boot - unplug power, wait, plug in. Rebooting from the web interface doesn't do it.

What could this be?

Thanks

maybe dhcp / dns issue?
try lan > general setup> tick the small box "Force router to use DNS server IP address" & then details page add the dns manually if helps
There is also a newer firmware 3.7.8 here
http://www.draytek.com/index.php?option=com_jumi&view=application&fileid=15&Itemid=583&lang=en
If you update use the *rst* file extension (that will set everything back to factory default everything) but it helps for a clean instal

Problems seems sorted after upgrade to firmware 3.7.8