Core issue
Cisco bug ID CSCse51804 documents this issue.
A DMVPN tunnel may flap at regular intervals on a router that runs Cisco IOS Software Release 12.4(6)T2. The Next Hop Resolution Protocol (NHRP) cache entry at the hub expires a long time before its expiration time.
Resolution
In order to resolve this issue, upgrade the router that runs Cisco IOS Software Release 12.4(6)T2 to Cisco IOS Software Release 12.4(6)T03 or later .