Wireless Network Problem

Hi I live with two roomates and I have set up a wireless network. Everything was working fine until a person in the nextdoor apt. set one up too. Now we occasionally lose our broadband service and sometimes end up grabbing ip’s from their router. I tried setting static ip’s for us but we still lose our connetion occasionally. Like I said everyting was fine until the other wireless was set up. Any ideas on how I can isolate this? Thanks.