328 Door Lock Issue (Locking By Itself)

If it's really a mechanical thing (the knob "sinking") then it sounds like a balance spring is wearing out, allowing the weight of the parts to move the mechanism. You'll have to get the door panel off to check that one out.

You might check for stripped wires, though, if it might be the power mechanism pulling the mechanical bits around when a wire gets grounded.

 

My driver's door does that when the latch mechanism gets dirty. I clean it annually with a graphite compound. (Graphite suspended in alcohol - lock cleaner from Ace Hardware.)

 

I had door locking problems on my Mondial t, perhaps the 328 follows a similar principle:
The problem will be with the door lock in the opposite door to what you think. If the drivers door keeps locking itself then it could be getting a signal from the pax side to say lock. This happens via a fancy mechanism inside the lock mechanism moving a little 12v motor. This signals the opposite door to lock. One way to isolate this is to disconnect the pax door motor from the plug inside the door trim and see what happens to the drivers door. If all appears OK then it is likely to be a problem with the little 12v motor inside the pax door lock. If you disassemble the door lock you may be able to sort it out or fit a new little motor ($2). If you pass it to a dealer they may end with the same solution but will replace the door lock ($300) plus labour.
The 328 may be a completely different system in which case disregard.
(Just had a look at the parts manual and the door lock mechanism for a 328 is the same part number as the Mondial t so the above story may be relevant)
Good luck
Terry