Thanks for the reply.
back up saved configuration file - now have made one.
Someone maybe resetting the router to factory defaults - When this was noticed I was alone in my home. Even if someone resets, I guess entire range of settings would get altered and not just Wireless portion.
someone is accessing the routers web page and changing something - That's what scared me..but at least logs did not show any new handshake (to what best I understand the language of those logs)
While router should not change anything just like that, any possibility that the RAM or whatever that holds the settings in router might have got corrupted etc causing portion of settings to get altered. Just a thought though. Last 2 days nothing unusual noticed as such. Shall keep a watch. Thx.