Sorry, I had no luck figuring out how to avoid this issue. All I was able to figure out is that certain devices are more likely to trigger this bug, and it usually happens after a long time or with a lot of traffic coming through.
WDS for example is a complete disaster in terms of stability.
I haven't really tested the future release of OpenWRT on my C2600. It's possible this might have been fixed already, who knows.
Also, if your C2600 keeps restarting at random times, see this thread: