If it got better when you sprayed it with WD-40, now you know where you have to spray it with real lubricant. WD-40 is not a lubricant, it's a cleaner that will seize your lock mechaism well and truely in a few weeks if you don't lubricate it properly. I would try one of the spray white grease products, and get all moving parts while you are in there.

Here is cold and salty Ottawa, these cars need the door panels taken off and the various mechanisms inside lubricated every 8 years or so for smoothe operation.

We had a similar problem with our 740, except that we discovered that the lock cycled perfectly with the door open. This suggested that the problem is actually failing wires at the door hinge. I never tested this, as the car was wrecked shortly after the problem surfaced, and the 940 hasnt got this problem yet. It should soon, though!