IMU Mismatch warning

I recently completed a clone flamewheel 450 build with a 3DR Pixhawk 1. I’m mostly very happy with it - everything I’ve tried works well. However I’m getting an IMU Mismatch warning on most flights and could use some help narrowing down the cause.

This morning I flew 5 flights back to back, and got IMU Mismatch numbers of 1.34, 1.41, 1.08, 1.14, and 1.04. In the past over about 20 flights I’ve had numbers in the range 0.65 to 1.41. It’s not getting better or worse, it seems to be random, within this range.

The recommendation is to check vibration and accelerometer calibration. I calibrated the accelerometer before flying. The vibration levels look ok to me, but I’d appreciate any insights on that. I’ve attached the five logs from this morning.

Why would the mismatch number change so much from one flight to the next?

I’ve seen forum references to 3DR Pixhawk 1s from the second half of 2014 having a hardware problem that can cause an IMU Mismatch warning. I purchased mine from 3DR in the summer of 2016. I’m assuming it would be manufactured more recently than 2014, but is there a way to check?

Thank you.

3DR Pixhawk 1
ArduCopter 3.4.6
Emax 2213 935kv w/1045 props
3S battery