@paul0000 : just a little comment on you last section mentioning that you couldn't see how cables would impact.
As you and I both know (since we both work in the IT business) LAN cables can always impact network stability, but indeed as you probably meant, this has little to no relation with wireless dropouts of individual TRV's.
However for the PSU part, I have actually had some experience with a few devices that had a faulty PSU (not able to provide the required amount of power). These devices (one in particular) had a PSU that was just on the edge of the requirements (even though it was provided with the device it self) and when the device draw a little bit more juice from the PSU then expected, the voltage drop of the PSU caused very weird behavior. The device did not reset/rebooted it self, neither did it stop working (which made it very difficult to debug), but some processes ended up working a little bit slower as well as some wireless power output dropped a bit (which defines wireless range). In the end replacing the PSU with a version that has a little bit more juice solved the problems.
So, also for everyone else reading this, although highly unlikely, it is not uncommon for a PSU to go faulty and cause problems difficult to identify.
The best option if you do want to test with another PSU, is to use one that has significantly more power (e.g. use a 2A or 3A USB PSU), preferably also with a good quality cable of course ;) and see if that helps. If not, then it is likely the original PSU is just fine.