Erratic Tach reading help

[chepburn]

Hi all,
I need some opinions on an issue with my tach sensor.
On x-country last weekend, after about 40 minutes of flight, my EFIS reported an erratic RPM (2800-4500) I have a constant speed prop, and since the engine didn't depart the airplane, its something at the sensor. So, cowls off, checked all wiring back to the EFIS. All looked good, although there could be a vibration short I couldn't see. The sensor is a UMA Tach pickup for the Slick mag.
Now, my question: The RPM log showed a steady reading until about 40 minutes into the flight on the way out, and the first 10 minutes on the way home. After that, the readings went totally random. My suspect is the sensor, or its wiring. But, since the bad readings don't appear until after everything warms up, could the sensor be giving me a warning signal for a mag issue? The engine EGTs and CHTs looked normal on both legs, and, there was no discernible rough running.

Anyone else have a UMA sensor fail this way?

I have already ordered a new sensor.

Thanks in advance for any help.