We dont know much about your system you did say it's 10 years old and...
Update: While I'm not ready to shut the door on this case, I think I solved the issue by buying a new bedside control pad for all of $12.95.
Frank
PLC's (PowerLine Commands have to be put on your powerline somehow (since it is random modules it can't be just a bad module or switch). If it was an "eye sensor" somewhere that maked it appear to be random... changing the House code would have fixed that also. Even a defective TM751, lost eye sensor, or a remote in the seat cushions would likely appear to be fixed with a house code change.
So with a new controller, and new house code(s)... I am lost for an idea without more information about what all is part of your setup.
My set up is actually very simple. I have one bedside controller, and four modules that operate various lights in the house. Some interior lights, and some exterior. Here is the inventory of modules. All are wall switch variety.
Channel 1: Front and back porch lights
Channel 2: Outside patio lights (2-way switch + a 1-way). The switches are inside
Channel 3: Bedroom (2-way switch)
Channel 4: Hallway (2-way switch)
I also have a plug-in module, a battery operated wall-mounted control pad, and a key fob, but I have not re-programmed any of those to the new code, so they are effectively non-existent.
I'm not sure this helps, but it's really all I have to go on.
Frank