Connect GCS and look at FlightData->PFD when you increase throttle. Does PFD bank angle jump? From good to bad or from bad to good? Probably moving throttle "from above zero to zero" PFD jumps ... and moving throttle to "from zero to above zero", PFD moves slowly (like servo moves).
If it jumps, you might need to recalibrate Attitude->BoardLevelCalibration or just redo all calibrations and see if PFD and servo work as expected. BoardLevelCalibration should be done with it level, not tail dragger sitting on tail wheel.
Unfortunately, some chicken and egg here. I haven't seen anyone set up fixed wing stabilization without first flying it manually.
I personally always get it flying in Manual mode, then add Rate mode, then add Attitude mode (adusting different things at different steps).