New Construction (typically) will roll up the extra LON cable and shove it back in the push button box, duct and/or in the controller. I've heard it basically becomes a transformer of sorts and creates missing nodes.

Also, I've seen dimmers and CFC light bulbs in the cab ceiling really screw with old Janus units and FCB's. I once had a car where the customer complained that the lights were flickering. I was checking the 120VAC cab light circuit and found read where it was 90Hz. They called the power company and while they were outside checking the transformers I disconnected the dimmer on the cartop and the problem went away.