Funny thing is I had similar problems a few weeks back. My Insteon/Elk setup has been flawless since the initial setup about a year ago with around 50 devices.
Recenty I added a couple devices and changes some scenes around using PowerHome and then for two nights in a row, random lights would come on?
After that, I unplugged the PLC at night for a few days until I had to look into it... funny thing is, lights didn't seem to turn on during the day.
All the Insteon groups/scenes have been configured with PH, and PH is also used to populate the Elk M1XSP. Aside from adding a couple Insteon devices, I also updated PH to version 1.03.4.12 (all the work had previously been done using a much earlier version).
What I did that seemed to correct the problem is connect my M1XSP to the laptop running PH and then re-downloaded the core app, cleared the PLC database and then used the "Add Min" to add the Insteon ID's back into the PLC. The only thing I did differently the day the problems started is to use the "Add Full" which I don't think was an option in the previous PH version.
Lighting has been reliable again for about 10-12 days since doing this.
One thing I noticed is the lights that were turning on were all lights that I use Elk rules to turn on with motion or door contacts? Don't know if this was coincidence, or a clue, but I think reloading the PLC using the "Add Min" seems to be what corrected it.
A few othere bits of data: I've had polling enabled on the M1XSP since initially setting this up. I've also been running Firmware 50.0.25 on the M1XSP but I see there is a newer .26 version that appears to have some good improvements acording to the release notes.
Cheers,
Paul