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I experienced this but it was during track events and the TCU thought I was abusing the transmission. (I wasn't). You might need to take it to Nissan/Infiniti. Their Consult reader can see trans codes that an OBD reader can't get at.

 

If that's your issue, it's different than what I reported.

 

Inside the shifter assembly there are three (3) micro switches as shown here:

When you slide the shifter to the left (to enter DS Mode) the DS switch closes. The Shift (+) and (-) switch are momentary switches that close when you slide the shifter to shift up/down (aka "Manual Mode").

You should be able to access these switches (for testing) by removing the center console trim piece. You do NOT have to remove the shifter from the car. I do not believe the contacts on these specific switches can be "cleaned" but at the very least you can test them with good multimeter and also check that the contact tabs are intact.

Hope this helps...

EDIT: to clarify the image above, the shifter is shown from the driver side of the center console.