I have a 8-year old Trane electric furnace. The issue is an intermittent chattering contactor switch which controls the heating elements. I have instrumented the 24v voltage on the contactor coil and the voltage drops when the switch is chattering. The chattering only occurs when the system is closed up, never with the covers off. The low voltage path on the contactor is transformer -> thermostat -> fan switch -> limit switch (155) -> contactor coil. In attempting to isolate the problem, every component in the above path has been replaced. In addition, the filter is new I have cleaned the evaporator coil, and checked that wire terminal screws were tight. The problem seemed to be fixed for a week, but my renter says the issue is back after a night where the temperature low was 18 degrees. The only thing I haven't replaced is the thermostat wiring, but that seems unlikely since the issue doesn't happen when the system covers are open. No issues with fan operation or the prior summer air conditioning. When I temporarily bypassed the limit switch (after the new one), the issue didn't occur but I may not have ran the system long enough for it to happen. What am I missing here in debugging this problem?