No sim / Invalid sim error

So every once in a while, my iphone 3g will show up with a No sim card installed message or it will say invalid sim. If I reboot the phone it works fine for a little while then does it again. My sim is totally legit, I bought the phone through AT&T, it's not refurbished or anything. My only problem is that my 1yr warranty is up so I can't take it back. Is my iPhone starting to die or is there any way this can be fixed?