Intermittent DMVPN tunnel flaps are commonly caused by reachability problems between routing neighbors or between the tunnel endpoints that support those neighbors. DMVPN depends on several layers working correctly: the underlay transport, GRE tunnel interface, NHRP registration and resolution, IPsec protection if used, and the routing protocol running across the tunnel. If the routing neighbor cannot remain reachable, the tunnel may appear to flap even though the encryption profile itself is not the root cause. A suboptimal routing table can produce poor forwarding or asymmetric traffic, but it is not the most common direct cause of repeated tunnel up/down events. Bandwidth congestion can degrade performance, but a correctly designed tunnel does not flap merely because utilization is high unless keepalives, NHRP, or routing packets are being lost. The fact that a GRE tunnel is not encrypted is a security design problem, not a tunnel-flap cause. The right troubleshooting posture is to verify stable underlay reachability, tunnel interface state, NHRP mappings, and routing adjacency stability before assuming an IPsec encryption failure.