What was found
FLIGHT 9195, ODOR IN CABIN BELOW FL250, WHEN ENGINES AT IDLE DURING DESCENT, WOULD DISSIPATE WHEN ENGINES CAME BACK UP. APU WAS OFF, ENGINE BLEEDS ON. ON PREVIOUS FLIGHT 796 DURING DESCENT CABIN AND COCKPIT AIR QUALITY DIMINISHED WITH AN ODOR SIMILAR TO DIRTY SOCKS. MAINTENANCE FOUND NO FAULTS, ACFT RELEASED FOR MAINTENANCE FERRY FLIGHT. MAINTENANCE REMOVED AND REPLACED THE APU. ALSO REPLACED THE LEFT AND RIGHT PACK HEAT EXCHANGERS, OZONE CONVERTERS, AND CONDENSERS. ACCOMPLISHED DECONTAMINATION OF THE ESC SYSTEM.