router drops settings

My Linksys WRT54G v.5 router sometimes loses all its settings, and goes back to the factory default settings. I reset everything, and things will be fine for weeks, then suddenly it happens again.
The router & modem are always powered down when not in use. When the problem occurs, it's always when the router is turned on, it never happens while it is in use.
I just upgraded the firmware, reset the router to default settings, as Linksys recommends, and entered my settings. I'm always hopeful that the latest firmware update will solve all my problems.
Anyone else have this problem? Thanks for any help

Thanks, I checked, remote administation is disabled. I'm using MAC address filtering & WPA2 with a strong encryption key, so I doubt anyone is getting in from outside the network, and I keep the login username & password in my head, but maybe I'll change them again just to be sure.
I'll wait & see if it happens again. If it does, I'll try to return it. Luckily, I bought it at a locally owned shop, they will be accomodating.

Well, I don't know, and I may never know. I always turn off anything when it's not in use to save energy, it makes no sense to leave it on all night or during the day when no one is around. The router & modem are switched on or off together. And since the problem is intermittent, it makes it very difficult to figure out what might have happened.