Thank you for your question, my name is Steve and I am a fully qualified vehicle technician with over thirty years in the trade. Hopefully I can help.
Whilst the problem may be due to a sticky drivers switch and this could prevent the passenger switch from operating correctly, the most likely scenario in my opinion would be that the wiring loom between the drivers door and the body has failed.
This is a very common problem and due to the wiring flexing every time the door is opened and closed, this can be intermittent.
I would check the loom between the door and the body which is routed between the hinges for any small breaks or cracks in the wire. If you have a rubber sheath over the loom you will need to pull this back to carefully check the wire, but i believe this is the most likely reason for the problem you have.
You also mention could it be a motor problem and again this is possible but not as likely as a fault in the wiring.
I hope this answers your question but do please feel free to ask any further details by replying to this thread.
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