On a failing DRO the frequency drift is most evident during the self heating initial warm-up. During this warm-up, you could observe the LO shift 5+ mhz in the first few minutes of operation.
The extreme of an ambient temperature adds the additional frequency drift that users finally notice. A high or low outside temperature is the last straw which causes the LO drift to exceed their receiver's AFT circuitry range and the tuner can no longer lock the preprogrammed transponder.