Have you tried upgrading the router's firmware?
This may not be the case, but if your router's second hand then it might be on its way out, or it may just be faulty anyway. Linksys' BEF series are known for their crashing and dying I'm afraid. I've already been through 2 (each lasted a year) and my third one's going as well.
I recommend doing some reading here:
http://www.dslreports.com/forum/linksys