What was found
Cabin continued to climb at approximate rate of 200-300 FPM after level at 35,000 feet. Crew initiated a rapid descent and performed COM procedures. Cabin pressure was controllable in manual mode at the lower altitudes. Flight diverted to MEM and landed without incident. Maintenance found a sense line for the right pack heat exchanger airflow disconnected and resecured it. Pack operation is normal and the aircraft has returned to service.