[MT-AU Public] Can't connect through Winbox/Dude

I have an intermittent problem where some routers stop allowing Winbox or Dude connections. At the moment I have a CCR1009 at a client site that's working perfectly and I can ssh into it, but I can't use Winbox or the Dude. If I reboot it, it'll come good (but I'll have to do that late at night). I've had some x86 routers do the same thing. I can't see anything obvious in the logs or active connections. The problem occurs with multiple different ROS versions. Does anyone have any suggestions for troubleshooting this? Thanks Russell