Central Locking problem

[Tornado]

My EL Wagon has just developed a strange problem with the central locking.

Sometimes when I press the remote to unlock the doors, the doors unlock then lock again. When this happens if I use the key it unlocks fine.

Also if I press the button on the dash to lock or unlock the doors, the doors lock and then unlock about three times then stop in the locked position. This happens when the engine is on. If the engine is off the dash button works fine.

So,
- Using the key always works fine to lock or unlock the car.
- The remote works fine to lock the doors.
- The remote only sometimes works to unlock the doors, other times it unlocks then locks them again.
- The button on the dash works fine with the engine off, but with the engine on it locks then unlocks the doors about three times in a row.

Any ideas what of what could be wrong ?