That radio thing had me tearing my hair out!
I noticed while testing several temp gauges on some Formula Ford race cars:
The gauge reading was affected by where the sensor TUBING was located. The first few inches of the tubing, coming from the sensor end, can dramatically affect the gauge reading. I heat wrapped the first few inches of the sensor tubing and, in some cases, relocated it to get a more accurate reading. I was REALLY surprised how much difference it can make. It IS subject to outside air temps, fan air flow on or off, DRIVING air speed slow or fast, header heat, etc.
I wonder as well if the gauge readings are being affected by a voltage change, lights on or off.