I've been having this problem too since I bought the routers, almost the same configuration you describe. It's a software issue and Asus says on another forum that they are working on it, they call it vlan loop:
http://forums.hardwarezone.com.sg/90967517-post1086.html
That posting is almost...
No loop
After all the messages about this problem I think it should be clear that this is not a simple loop. And spanning tree protocol does not solve it, believe me I've tried.
I agree with eNORm, can Asus solve this once and for all. Too many people are having this problem.
Thanks.
Eric.