in mine, it'* the solenoid in the door panel itself. i'm guessing it'* built up old grease. on a really hot day, it will lock and unlock like champ, but most of the time, it will only lock. sometimes, on colder days, it won't do jack.
sooo, again, i'm thinking it'* warn out grease that they pack in the solenoid. as it heats up, it becomes, well, more greasy
, and allows the solenoid to move freely, but as it cools back down, it hardens up, and prevents the solenoid from doing it'* job.
anyway, replacing the solenoid would take care of that. and if it'* only the grease, it could prolly be re-greased.