What was found
INFO ONLY: DURING CRUISE AT 390FL AT ABOUT 6 MIN INTERVALS, THE OUTFLOW VALVE WOUD MOVE FROM A QUARTER POSTION TO HALF POSITION CAUSING THE A/C TO SHUDDER. IT HAPPENED CONTINUOUSLY. PERFORMED CABIN PESS TEST, IAW AMM 05-51-24, SYSTEM TEST OK. PERFORMED MANUAL MODE CONTACT TEST, IAW AMM 21-31-00, TEST OK. PERFORMED BITE TEST ON CABIN PRESSURE CONTROLLERS, NO FAULTS FOUND. A/C OK FOR SERVICE.