Does it not make you suspicious that there is a discrepancy between the two readings? Perhaps that points to the problem? Seems hard to believe the tach on the dash could be that far off....

here's another idea.

Scott, I was actually rereading your post from the beginning and it struck me that you have the traction control defeat switch installed in your car. This is an electronic mod that was not stock.... have you inspected the switch mount and all wiring related to this mod? When we swapped modules a while back, i noticed
some of the wires under your computer housing had been unbundled, whereas they were neat in my car. Is it not possible the SRS defeat switch was spliced into some of that?