phone will show reception then lose reception rendering it useless

I have an iphone 3gs approx. 8 months old. One day out of the blue it had no reception. Couldn't make a call nor receive one, couldn't send or receive texts either. Tried a couple of manual resets with no luck. The phone would however start randomly getting reception for short periods of time and then go back to nothing. I have had it sitting in a drawer at work for about a month or so and my wife damaged her phone so I thought I'd give it another try. I reset the phone back to factory settings, added her sim and restored from her last backup. Viola, everything pulled up great and phone was working perfect. It's been about 3 hours and now it is back doing what it was doing before. Any ideas what to look for? I really need to get the phone working to avoid buying another to replace my wifes. Thanks!