Thank you for the response Dirk

I did some more testing yesterday. I fully charged my Rx packs and found they were half charged (two 2s 3000mah Lipo). My PowerBox is set to regulate to 7.4v as I'm running HV servos; since the PowerBox only has the option for 7.4 or 5.9 and the ECU needs a minimum of 6v, it's now my only option with my current set up.

With two fully charged batteries everything was a- okay. If I powered down one battery the issue would return. If the batteries are only half charged then even with both turned on the issue would persist. I kept a watchful eye on the regulated voltage as you had mentioned the 6v threshold. What I found was that as soon as the voltage dropped below 7 volts it would start exhibiting the noted behavior.

The last thing I want is for a tach error to shut down my engine in flight, especially if one of my two Rx batteries happens to fail or if I want to fly more than once between charges, or if the power consumption in the air is high enough even with everything perfect to momentarily dip the voltage below the 7 volt threshold.

What are my options? I could, and maybe should, get new, better, receiver packs; but even then with a 7v threshold it seems like the error could still occur. Would it be feasible to separate the signal lead on the Rx in on the ECU and run a dedicated battery? Not ideal since it would add weight.

Thank you