Hi David,
Yea not to preclude the use of the DIMU, just an idea to perhaps narrow down the problem. I had issues with an early DIMU prototype and code (needed more filtering on the GYOs to fly with PID controller), but a lot has changed since then. You don't have to dismount the DIMU board to try it. But I'd start with fresh default settings.
I have no idea about the attitude deviation, sorry... never seen that. My initial suspect would be calibration or perhaps vibrations. What does ACC Magnitude graph in telemetry view show when you move the board around to different orientations?
-Max