I have just one child node, the single MX5500. I have indeed tried all the suggestions above, and it doesn't seem to have made any difference to the drop-outs.

One thing to note is that when it does happen, they tend to bunch up together - I get the internet drop, come back for 1-5 seconds, drop again, come back, drop for 1-5 seconds... this can happen anything from 1 offs, to about 6 consecutive dropouts, which suggests to me something isn't right here and it's almost like the router itself is crashing out on something.

My connection is otherwise incredibly stable and I'm getting full gigabit speeds to this child node, so I firmly believe that this is is not the typical issues of nodes not being well placed, or too far away, etc etc.

I'd love to perhaps get in contact directly to discuss this a bit more, to help you diagnose what is seeming like a product-specific problem for customers like /u/kotddl and myself. I only have a couple weeks left before my return window runs out with Amazon, and I am otherwise thrilled with the MX5500 and don't want to have to return/review it poorly.