I have a Deco Mesh system in my home consisting of 3 Deco x50 and one x20. Fibre to the property, 500mb speed, main deco with satellite decos connected via Ethernet. 
 

They've been working fine (mostly) for a year or so, and are running the latest firmware. 
 

However over the last few weeks the internet / WiFi is really unstable. Pages / apps / steaming take ages to load and then work for a time; then buffer; disconnect; then connect; then work, then buffer..... and so on.

 Same issue on on iPhones, TVs, everything. Wired connection doesn't seem to be affected just WiFi. 
 

Tried running an optimisation and rebooting countless times but nothing works. It's as if the signal has gone really weak, devices dropping out etc.

For example my doorbell always worked fine but now drops out daily. Bedroom TV WiFi buffers constantly, iPhones take ages to load pages then when they do load I get decent speeds.

Anyone got any advice? 
 

We had a power cut about a fortnight ago and the issues appeared to start the same time, but could be coincidence.