Do not use a IVR with aftermarket gauges. The IVR drops the instrument voltage down to ~5volts, and aftermarket gauges require a full 12V. If your aftermarket gauges are bouncing around, you may have a bad connection or a short somewhere. Get a hand held volt meter from Radio Shack or elsewhere and check the voltage going to the gauge.
Dynacorn 67 fastback. Dart 363, Close ratio Magnum 6 speed, 3.70 Eaton Truetrac, SorT suspension. Still in pieces.
'14 SHO with most all the bells and whistles. Stock for now.