What was found
DURING CRUISE FLT FL 380 L PACK SLOWLY DROPPED TO 10 PSI FOLLOWED BY A RAPID INCREASE TO 33 PSI CAUSING A PRESSURE SPIKE. THIS OCCURRED NUMEROUS TIMES. DESCENDED TO FL 250 AND SELECTED ENGINE ANTI -ICE DURING DESCENT AND PROBLEM WENT AWAY. OAT FL 380 -60C. ACFT GROUNDED. R & R PRSOV IAW TASK 36-11-04. OPS LEAK AND CHECKED OUT GOOD. R & R BLEED AIR REGULATOR IAW TASK 36-11-03. OPS LEAK AND CHECK OUT GOOD. PARTS VERIFIED BY MX. ACCOMPLISHED REVERSERAL OF ALT DEACTION PROCEDURES, REQUIRES FLIGHT OPS CHECK VERIFICATION. VERIFICATION FLIGHT SUCCESSFUL.