An Easy VPN tunnel might flap due to many reasons. These reasons include a line condition or a hardware issue. A tunnel can even go down if it sits idle for more than the specified time or because of stale security associations (SAs) and so forth.
Use the crypto isakmp keepalive command to isolate this issue if a tunnel flaps because of the SAs lifetime or if it is sitting idle.
Info regarding "crypto isakmp keepalive":
This command allow the gateway to send dead peer detection (DPD) messages to the peer, use the crypto isakmp keepalive command in global configuration mode. To disable keepalives, use the no form of this command.