Some say the cables should not be lubed, some say use a dry lube. However, I agree that 50F should not be out of the operating range. Maybe your bike is telling you to pedal around sunny San Diego.
You might want to pull the wires from the housing and inspect. Perhaps someone greased the wires and combined with dirt, formed a temperature dependent goo. You may want to clean the wires and insides of the housing to remove any goo. But since you have everything stripped down, perhaps it's time for new wires and housing(?). Also, check the pivots on your der - make sure they're lubed and travel smoothly.