I had similar symptoms and it ended up being that when I upgrade from the R70 beta to R72 via the web ui, the rulesagent process would go nuts and use all the ram and the router would crash.  I had to revert to R65 (only version I could find in a downloadable).  I also tried the hard reset and then a manual flash to R70, but still had the same issue.  Currently I am stuck on R65.