What was found
AT FL 400, CABIN CLIMB RATE INDICATOR STARTED TO FLUCTUATE 1000 TO MINUS -2000 EVERY 2 TO 3 SECONDS. AFTER ABOUT A MINUTE SWITCHED FROM AUTO TO ALTERNATE WHICH MADE NO DIFFFERENCE. DESCENDED TO FL380. FOR ABOUT 3 TO 4 MINUTES NO CHANGE THEN. STARTED TO IMPROVE TO 500 TO -1000 FOR ABOUT ANOTHER 3 TO 4 MINUTES AND THEN STABILIZED TO -0 AFTER A COUPLE MINUTES. TESTED MASTER CAUTION RESET AND NO LIGHTS ILLUMINATED WITH RECALL. ACFT GROUNDED. PERFORMED CABIN PRESS CONTROLLER 1 AND 2 BITE CHECKS REF. FIM 21-31 TASK 801 NO FAULTS NOTED. PERFORMED PRESSURIZATION SYS GROUND TEST REF AMM 21-31-00 TEST CHECKS GOOD. ALL GROUND VERIFICATION CHECK GOOD. ACFT OK FOR SERVICE.