Either one or both of my rear doors refuse to lock with the central (power) locking using the button on the key and so I have to lock them by hand. This varies as sometimes it's just the one door and other times it's both rear doors. However, Once I've locked them they unlock normally with the rest of the doors. If I'm in the car I can use the driver's lock button to lock the doors, but the rear doors only lock when I press the button twice in quick succession.

Also, when turning the key in the ignition, either one or both of the rear doors unlock even though they're usually unlocked anyway.

Any ideas?

Make sure the exterior door handles are completely in. My '96 had this problem with one side, and it seemed to be related to the door handle not retracting quite flush. I removed the handle (a simple operation -- just unscrew the large philips-head screw above the latching mechanism), lubed the small internal return spring inside the handle, and now the handle snaps back flush and the lock works as it should.