1997 ford fiesta doors wont open from the outside?

Hi, for some time now the lock on my 1997 Ford Fiesta has been playing up. Occasionally it locks and unlocks by itself, I could live with that but now the doors will not open from outside, both sides and I am having to climb in through the hatchback. I can open the doors from inside the car although the drivers side is really stiff, I am having now to leave a window slightly ajar so I can get my arm in to release the handle. Help