What was found
FLT 1745, AIRCRAFT PERFORMED AN AIR TURN BACK T BOS, CABIN PRESSURIZATION CYCLING UP AND DOWN AS MUCH AS 2500 FPM DURING CRUISE. MANUAL CONTROL OF THE OUTFLOW VALVE, NO HELP. BLEED PRESSURES CYCLING FROM 10 PSI TO 48 PSI. BOTH BLEED CYCLING PRESSURE BUT MORE SO ON RIGHT. REMOVED AND REPLACED ACM IAW AMM 21-51-05 UP TO ITEM, SUBTASK 420-002-A STEP 9, 3582714-2, 3582713-1. REMOVED AND REPLACED DUAL HEAT EXCHANGER IAW AMM 21-51-04. OPS CHECK AND LEAK CHECK REQUIRED, 3582713-2, OPS CHECK AND LEAK CHECK IAW AMM 21-51-00.