All of a sudden, a handful of insteon lights switch on by programming, but then switch off spontaneously in a second. Have checked all programming and nothing has changed. Have tried reboot but problem persists. Any ideas?

Handful? Like a scene maybe? Always the same ones?

Almost all (about 12)

None are in scenes.

Same thing happened to me a couple of months ago. No record in the logs.

I did a power cycle on the eisy and also did a link table diagnosis/compare on all the affected devices. Did a restore device on any mismatched link tables. All is good now.

Also take a look at all your programs to make sure you don't have a corrupted command(s)