@lazy88 

I would advise when you notice it come back onto your network quickly get it blocked. 

Then you maybe might notice that if it is one of your devices that it won't work. 

Say like a alexa device ect. 

Can you not check your devices by their settings or via their respective apps to see if it is one of your devices ip/mac address. 

In my home setup I've got erhernet ports all around my house, 

If I can have my devices wired they are so you can get double ip/mac address for the same device. 

I.e my tv is hard wired but it also has been attached to the wifi,

so if you have this In  your house maybe a devices is getting confused and is sometimes connecting via wifi or etherent Which might be the reason your getting a random connection or you could have a connection that is going faulty say hard wire and your wifi is then kicking in showing a new connection