@walkinthepark 

When speed drops to 90/25, open Deco app and you should see "Abnormal WAN speed" message.

What you describe points at issue with Ethernet cable between Main Deco and NBN NTD. On Main Deco restart or cable unplug/replug gigabit speed is negotiated over that cable, but for whatever reason can't be sustained, so both sides renegotiate speed to 100Mbps.

Replace that cable with different one. 

  @Alexandre. 

Thanks for reply.

Yes I had seen that message before on App while speeds are down. Hadn't thought of that cable as it was brand new not long ago. 

Will try that replacing and see how it goes