For those of you REALLY following all of the details. The accel was likely overloaded on its input due to a typo on a spec sheet (and my fearless experimentation). I knew something was wrong when it quickly shot up to 175+ deg F
. All of the data in the OP was taken after the overload. The frequency resolution seemed unaffected but it did have a zero shift and small change in output which I accounted for in that data. I have sent it off for repair/recalibration or replacement. There will be no additional testing until I get the unit back. It will probably take 1-2 weeks.