BlackBerry 8800 network registration problems, any solutions?

Alright I'm starting to get really frustrated, my BlackBerry 8800 keeps unregistering from the BlackBerry network. I've called AT&T several times, and I get the same answers from those on pushers and nothing ever seems to work. I've ran the diagnostics program several times, changed host routing, rebooted the thing, data on/off, just about everything! Any insight into a permanent solution? It seems to do this everyday in the evening and I don't know why. Thanks in advance!