Help Full signal to no bars or roaming


I live in a city where Sprint coverage is great. I just got my Evo and I am constantly never get more than 3 bars and roaming. It goes from 3 bars or full signal to roaming every time I try. I just replaced my Evo because the one I had was doing the same thing. I just got it yesterday and havent done any mods to it. I updated, PRL, profiles, and firmware but I still get this. I couldnt download any apps yesterday but while I was sleeping they all downloaded. It just took about 8 hours lol. Sorry for the long post but I am having the same problem today, going from x bars to roaming or no connection. Just wondering whats going on?