TP-Link TD-W8970 ip layer disconnects

I have one of these routers SQM (cake/codel) turned on. If I saturate the connection by triggering lots of downloads everything works fine for some time.

At some point the router will drop all connections (only TCP connections - the ADSL2 connection stays up fine), this is associated with a brief and sudden spike in CPU. Nothing else is happening on the router at the time.