My passenger door did the same, I replaced the small spring, greased up the lock etc and its been good.
Easy to do the spring and not expensive,get them on ebay.
Hope it helps
Try replacing the little round spring, i've got a rear door that does exactly the same but havn't got round to sorting it yet. If you unlock and then go round to the passenger side and reach through and lift the drivers lock pin up you should find that the door will open first time.