I've noticed a pattern but I'm not sure if it's a crappy router, crappy wireless device, or maybe everything is working as intended.

I can wirelessly connect my office PC to the router's settings page for a day, maybe two, then afterwards I can't anymore until it's reset.

Unrelated, but similar, in various places that offer free wifi, my kindle will connect fine most days, but occasionally won't unless the router is reset. After the reset, I connect ok.

What's happening behind the scenes that makes a router stop allowing you to connect? Is it something on my end that's changing, or on the router's end?