I'm having the same problem on my RT-A68P. This device has been running well for several years, but starting a few weeks ago I started having these problems. Here's what happens: sometimes I'm unable to log in to the web interface and sometimes when I do log in, it will freeze shortly after. When I am able to log in, I can sometimes see that both CPU's are pegged at 100%. I can usually SSH into the router, but then the SSH interface hangs. I'm on v384_12. This has been happening since at least v384.11_2.
Since this is a relatively recent problem, I'm wondering if it's a memory leak that was introduced within the last few releases. I have a USB drive with a swap file, but I don't know if that makes a difference if physical memory is filled up.
I'm tempted to just set a periodic reboot, but I would rather diagnose the problem before I use a sledge hammer approach to fix it. Can anybody recommend a way to diagnose this kind of problem? Are other people having similar problems? How many people use a scheduled reboot to fix this?