What was found
LEVEL AT CRUISE AT 34000 THE CABIN CLIMB GUAGE SHOWED +/- 500 FPM. LEFT DUCT PRESSURE FLUCTUATED FROM 45/40 PSI. R DUC WAS STEADY AT 45 PSI. LEVEL AT 24000 CABIN CLIMB SHOWED +/- 300 FPM. GROUNDED - YES. DURING CHECK FOUND #1 TCV FLUCTUATING (OPEN/CLOSE) CAUSING +/- 200 FPM CABIN ALTITUDE REF FIM 21-33 TASK 802. REMOVED AND REPLACED L/H PACK TCV REF B737-900 AMM 21-51-10. OPS AND BITE CK GOOD. NO LEAKS. NO FAULTS ON DCPC #1 OR #2. REPEATED AIRCRAFT CABIN LEAK CKS REF AMM 05-51-91. NO LEAKS. OPS CKS GOOD. NO CABIN ALT RATE FLUCTUATIONS.