mine does this all the time, there is a classic picture of me with my spanners and socket set out at a service station on the way to fcs!!
to the point, it is the solenoid that is the issue. best bet, if it does it occasionally (can be tested by locking and unlocking door with key then using fob) get some WD40 in there and loosen it up.
if not, new solenoid is the way forwards, dont think they are too expensive from a scrappy's, best contacting joshydee on here (cliospares dude)
jamie