there could be a possibility of network issue, if the phone keeps on resetting.
check the cabling of IP phone/switch port.
perform a factory reset and try connecting the phone on a different switch-port.
what do you see on the Phone Status messages and do you see any reason code in the ccm trace next to phone registered/unregistered.
CCM trace should give enough information on phone registration issues, however if you couldn't get much from the ccm trace, try collecting the Packet Capture from IP Phone, refer below doc for reference, also you can debug tftp events from the switch.
https://supportforums.cisco.com/document/44741/collecting-packet-capture-cisco-ip-phone