I have a blackra1n (tethered) jailbroken 3G running 3.1.2. Sometimes, when I leave it alone for a while (ie. at school in my pocket) the cell service dies. It just shows 5 flat lines. The wi-fi, 3G, and EDGE still work though. I've tried almost everything. In and out of airplane mode, respringing, deleting recent cydia installs, but nothing seems to work. Anyone else have this problem? or a solution? Tell me if you need more info.