What was found
ON INITIAL TAKEOFF ROTATION THE DFFERENTIAL PRESSURE MOMENTARILY STUCK AT ZERO AND THEN JUMPED TO 3.6 PSID WHILE THE CABIN ALTITUDE CONTINUED TO CLIMB NORMALLY. HOWEVER PASSING 15000 FT CABIN ALTITUDE EXCEEDED 5600 FT WHICH WAS SET IN CABIN ALTITUDE WINDOW. DIFFERENTIAL PRESSURE STOPPED AT 5.9 PSID WHILE CABIN ALTITUDE CONTINUED TO CLIMB. AFTER LEVELING OFF AT 25000 FT CONSULTED QRH RED TABS AND STARTED A DESCENT. ON TAXI IN VERY LITTLE AIRFLOW FELT FROM THE EYEBALL OUTLETS WITH THE GASPER FAN SWITCH ON. R&R CPC PER AMM 21-31-010-4 OPS CHK GOOD PER 21-31-000-2.