asterisk users - Sep 2004 - Cisco 7960G SIP Registration Timeouts

Hey gang,
 I have a 7960G in SIP mode. When the phone powers on it registers fine with
* and calls can be made to/from the phone. But after a while 'sip show
peers' no longer shows an IP for the phone. I can still make outgoing calls
but can no longer receive incoming calls (because * has no IP to route the
call).
 Any ideas on this? One of my other phones (MGCP) does a 'keepAlive'
packet
request every X seconds and so that phone is always working. How can I get
this SIP Cisco to do the same?

Thanks,
Matthew