I have 4 AP906 devices all using mesh.  Using the Draytek wireless app on my iPhone, it shows each device as blue (excellent strength).  After some time, node 1 may then change to grey (weak strength) or even become disconnected (red).  It just never seems stable for very long.  What's your experience?

The house is old (thick stone walls, no wall cavity) and wide (but not very deep from front to back).  Node 0 (the root node) is plugged into my PQ2200xb (which is plugged into my 3912s with a Draytek 10Gb SFP) and mounted on a wall about 6 1/2 feet up and one end of the house.  Node 1 is the other side of the room on the other side of the wall node 0 is attached to.  Node 2 is in a room futher into the house.  Nodes 1 & 2 are sitting on top of book cases, about 5 feet tall.  It is not possible to wire these APs.
Node 3 is perpendicular to the house and is in my garden office, using a wired connection directly into the PQ2200xb and doesn't really come into the equation (I believe).

Sometimes a power off of node 1 (leaving off for a few minutes, often 1/2 hour when I get distracted...) will bring it back to being blue, sometimes only green (good strength).  Sometimes I can do this process many times without making things any better but a reboot of node 0 will reset it and node 1 becomes good again.    Due to the "straight line" nodes 0-2 are in, I can't see any reason why the uplink of node 2 wouldn't be node 1, and the uplink of node 1 would be node 0.  Sometimes (most of the time), that is the case, sometimes however node 1 has an uplink of node 2 which has an uplink of node 0 (which makes no logical sense at all other than if node 1 is having issues so the mesh reconfigures node 2 to use an uplink of node 0 as it's faster, but why node 1 would then have an uplink of node 2 makes no sense).  Sometimes turning off node 1 entirely seems to make things a lot better.
All the nodes are set to use "auto" channel (partly because one of our neighbours seems to reconfigure his network on a fairly frequent basis!).
I'm aware I can set the uplinks on the nodes explictly but I'm conscious that if there is an issue with a node, this hardcoding may cause the meshing logic to stumble.

It's possible that the Draytek app isn't reflecting reality and the signal strength isn't great.
Using an app on my phone at various places, I usually see at least 160Mbps which is fine for my requirements so I'm not looking to improve that until I've acheived stability and I've run out of other things to do.

There's no logging of the meshing process, reconfiguring etc. as far as I can find, nor any technical documentation (which maybe isn't a surprise as it could be considered IP).  It seems that the meshing process is buggy and not very tolerent but I'd be interested to hear of others experience, including experience of other vendor's meshing stabilty (I'm not looking to migrate from Draytek - I've invested far too much to do that and frankly I like it).
If anyone has any ideas of how to debug the issue, I'd be very grateful (I'm not precious so don't hold back from basics if I've not shown I've already considered them).  I could certainly raise a suport ticket but it seems that the experienced Draytek engineers are too thinly spread to be able to spend a lot on a somewhat nebulous issue like this.  If lots of us are having similar instability, that would paint a different picture to if it's only me (or (for example) only people with AP906's etc.).

Thanks!