D-Link router resets itself

i have a D-link DSL 2640t wireless router. i have my network set up with a Linksys router giving out separate IP ranges effectively splitting the one connection into a public wireless network and a private LAN network for our offices.
my problem is that my D-link router constantly resets itself to the factory settings without any prompting from myself, and i have now come to discover without any power cut! i set it all back to the way it was supposed to be this afternoon and now 3 hours later it's gone back to factory.

is there a setting i'm missing? is there some way to make it stay how i want it? or is it a faulty router?
let me know if you want more info.
thanks for your help

 

sorry i realise now from other equipment that has turned off that there must have been a slight power outage, though i don't think that that should reset the router to factory settings!