Sometimes routers are just a pain in the butt. Sometimes a router can not handle the load its placed under(I am looking at you wrt54g).
Most times this kind of thing is solved with a firmware update. Or IOS(Cisco's operating system for its higher end stuff. The stuff that runs the world) update for bigger Cisco routers/switches. If you have a large switch/router It is generally more work to update its operating system(and done with hyper terminal last time I had to work on those).
Please post the model of this Cisco product you keep having to reboot.