As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THIRTY MIN INTO CRUISE FLIGHT, AIRCRAFT PRESSURIZATION BEGAN 1500 FPM CLIMB, FOLLOWED BY 1500 FPM DESCENT. INITIATED EMERGENCY DESCENT AND DIVERTED TO FSD. FLEW AIRCRAFT TO ILN UNPRESSURIZED. PERFORMED ADJUST/TEST OF CABIN PRESSURE CONTROL IAW DC9 MM 21-32-00. OPS CHECKED GOOD.