1. Make sure you are using full 3.0 not a RC
2. Uninstall (don't just disable) any unused packages.
3. Upgrade the firmware for the actual board (not just ROS)
I remember times when i bought WAR boards from Valemount networks. The boards were completely unusable, locking after few hours itsel, making a lot of troubles and when i tried to solve the problem i was getting answers like this, sometime i got answer about wrong power supply etc . I switched to Mikrotik and RB's.
Upgrade will not solve this neither firmware upgrade - for me there is some kind of bug in kernel code + unstable wireless drivers. I think when you are not pushing the hardware to higher speeds, this bugs are not present, but try to run it in live environment, over 10 Mbits of real traffic on wireless interface and you will see after few hours. But for lower speeds i dont need to use new RB's .
Sorry but, its better that a lot of people will report this bug , then maybe it will be solved. But then a lot of people will be unhappy and trying to "solve" the problem by config but i dont think this is a way for product which i paid - also the config will NOT solve the problem completely. The v3 is in early beta - not stable and can cause lot of people to think about stable systems like Canopy.
Jan