My EGT gauge sometimes will "spazz out" when I put the A/C blower motor on 4. It'll set to 0 degrees and then go up shake all over the place like the truck is vibrating really badly but if I put it down to 3, it goes back to normal. Any clue what this could be? Bad ground or something? Even if it was how would it have anything to do with the blower motor setting?

 

Where did you pull power from for the gauge? My tranny temp did that a couple times but it was because the wire was loose. My thoughts are maybe you tapped into the blower circuit and when you turn the blower on it pullse enough current to interrupt the gauge? Maybe try a different power source? I ran a wire straight from the battery to use for my gauges.