Well 55 ohms sounds good with front doors only. You can measure PSC/NSC resistance to ground. With all boards and FAST disconnected, should measure 1Mohm or greater when measuring each line to ground. Also with everything disconnected, measuring across PSC to NSC the cable resistance should be about 150 ohms. If all that is good, then you still may have a board or voltage issue. You can measure the 5VDC on the CLC board on the top left corner of the board across D72. Also, you don't have old & new boards mixed? Either they're all green or all blue?