Hi, please let us know if the new PLM is making things better once you receive it. I am experiencing exactly the same issues (even if I only have one Insteaon device for now). Everything works fine then suddenly, I start having trouble talking to my Insteon switch which is dual band and 4 ft away from the PLM.
I noticed that my UPS seems to have an effect on the issues, because when it is disconnected, it seems to be always working. It is not connected to the same outlet, but it is close to the PLM and the ISY so maybe it emits RF that messes up either themPLm or ISY.
I also noticed that when I turn my Insteon switch on from the admin console, and that it doesn't work, I just need to send an X10 command right after and suddenly, the Insteon device responds to the command I had previously sent. Really weird.
I will continue experiencing by getting the UPS out of the equation, and moving the PLm far away from it. I might also consider plugging the UPS into a filterlinc module to see if it improves everything.
So please let me know if you find a solution!
Thanks,