If in the logging you are getting the error message
%TUN-5-RECURDOWN: Tunnel0 temporarily disabled due to recursive routing
It means that GRE Router tunnel has found a recursive routing problem. The symptoms of this issue include the continous flapping of tunnel interface as well as EIGRP,OSPF & BGP neighbours when neighbours are over the GRE. To resolve this issue you need to configure the static route for the tunnel destination.