Downgrade to 3.XI have the same problem with all 4.x firmware versions. If the traffic raises over a "critical point", then the link disconnects.
With 3.x firmware, icmp gets higher at this point, maybe there was packet loss, but the link was still up.
The Problem is, that we need to use the 4.x firmware, because there are n-cards on the routerboard.
Does anybody knows, how to solve this?