Routers overheat and lockup, they get worse as they get older. Some do it, some don't. Of all the routers I've worked on, or owned Linksys has been most sensitive to this. They had an entire series that lacked proper heat sinks and most of those died in the few moths of ownership.
This has been kind of going on with Linksys for years now, people do all sorts of things to try and help, like this:
WRT54G - Adding a fan - dslreports.com
To answer your other question, unless your doing a lot of traffic on the LAN (between machines on your network) you're going to be limited by your external connection as mentioned above.