I had the same problem, took the door card off and it was the outside handle mechanism sticking, thought it might have been latch related as i had previously replaced the spring, but all i did was spray some WD40 on the other side of the handle all round near to where the barrel for the lock is, it then snapped open as it should, you'll be surprised, spray all round the pivot for the mechanism for the handle, on the inside, then pull/release the outside handle, do this a few times, it will work a treat.