May I ask, what makes you so sure about it? What tests have you done? Is it possible, that the SIM card holder became lose? Have you already tried if it's the same with another SIM card? Have you tried the SIM in another phone? Have you checked for dust inside? I recently bought a second N900 on Ebay. Everything's fine with it, but it was really dirty inside, so it's not impossible at least... Have you tried reflashing the device? Sorry, if you wrote that somewhere, but I haven't found anything like that yet...