Graceful degradation when the WAN pipe(s) have greater capacity than the router

Background configuration:
Balance 30 Router
1 cable modem (180 Mbps down)
1 DSL modem (25 Mbps down)
1 Cellular (4-5 Mbps down)
2 PepVPN connections

Problem statement:
When running full throttle the WANs overrun the capacity of the B30 (200 Mbps > 150 Mbps, the rated capacity).
As a consequence, the B30 functionality is impaired - DNS requests are not responded to, and it seems to stop routing altogether. It responds to pings.
When the traffic drops to zero Mbps the router does not recover.
The B30 has to be rebooted to restore the full functionality (waiting for an hour or more does not allow it to settle down).

Modifying the QoS settings to try to limit the speeds to below 150 Mbps in the aggregate did not solve the problem.

Feature request:
A router should degrade gracefully under load - that an incoming pipe can overrun the router and reduce it to complete non-functionality is not (ahem) a good or attractive feature. I have not experienced this with other (even consumer grade) routers running off the same pipes.

Suggested approach:
Make available a QoS profile that limits the overall bandwidth when the WANs start approaching the capacity of the router (i.e., 150 Mbps for the B30). When invoked it would not allow the user to try to turbocharge the router (I have seen the B30 occasionally handle busts of 180 Mbps without dying), but would secure stability of the infrastructure.