What's wrong with my brother's car central locking?

Besides it being a ford escort, that is... that's pretty obvious.

Had a look around for the answer to this on here and elsewhere on the net, nothing all that suitable popped up.

Usually it will lock just fine, and you can deadlock it / set the alarm etc. But intermittently, it will refuse to unlock. The symptoms seem to change every time, and it's very annoying. We're supposed to be running in it's new gearbox (yes, that blew up as well!) for him whilst he's living away in London, but don't dare go further than the local high street in case we either get locked inside, or can't get back into it after walking away.

Sometimes it seems just as if the lock barrel is sticking before it gets to the activation point, other times the system just goes wild, continually unlocking and relocking itself noisily... if you time it right you can get in through the passenger door, whereupon it goes absolutely crazy.

Last time, I had to clamber in through the rear hatch, tripping the alarm..