Autometer EGT interference issue

[procharge94]

I installed an autometer EGT in my '94 GT tonight and noticed something odd in the behavior of the gauge. When the motor is on and running, the needle in the gauge points straight down and wiggles all around. BUT, when the ignition is turned forward, but the motor is not cranked, the needle reads the correct exhaust temps. I re-checked the wiring and even changed the grounds, but still did not have any luck. Does anyone have any experience with these gauges or possibly have some knowledge as to what my problem may be. I'm assuming the motor running is causing some sort of interferance, but do not know exactly how and what needs to be done in order to compensate for this. As always, thanks again. I almost forgot, for positive 12V feeds I used wires from the existing stock radio harness.
Matt