Airplane:  Legacy RG / IO-550 / G900 / TruTrak Sorceror

Symptoms:  
1.  Pitch oscillations about selected 12,000' altitude:  +/-60 feet, +/- 2.5 deg pitch attitude, +/- 250fpm ROC, period 20 sec.
2.  Not as bad at lower altitudes.
3.  No other issues -- lateral, climb, descent, glideslope tracking unaffected.

Debug - (credit Zack at TruTrak)
1.  Confirm proper pitch settings on autopilot.
2.  Disconnect static at autopilot (feed cabin pressure to autopilot static input); problem disappeared.

Cure - blow out static system from inside out using 90psi compressed air.  Problem solved.

Caveat:  When disconnecting the static at the autopilot, plug the connection to the static system (OK, in retrospect:  DUH!!!)  Otherwise, you get erroneous airspeed indications (high) which introduce unnecessary risk.

Lesson:  The Sorceror suffers ill effects from static system lag that doesn't show up on the altimeters, which tracked each other perfectly.  The size of the error increases with altitude as the pressure gradient (dP/dAlt) decays exponentially with altitude.

Remaining mystery:  What was in the static lines?  Nothing visible blew out, not even water.  Maybe this will reappear.

Charley Brown
Legacy RG #299   200 hr