If you you have never ever Netinstalled this device not even for the changing to v7 series (which is the risk averse way to go) than it is definitely time for
/export verbose show-sensitive file=filenameyoulike
and Netinstall latest stable or testing (choose whichever you like) and
/system/routerboard/upgrade
. This will likely not only leave you with a bit more free storage space however might as well cure seemingly inexplicable issues such as the ones you are experiencing (for example same device with similar usage pattern works rock solid for some while often reboots for others).