The IGH cable stretches over time; it's possible that if the cable is too slack for
the IGH to go in and out position/gear. Since this doesn't cost anything; I'd try
this first.

Do you have a chain wear measuring tool? Or maybe ask a shop to do it, there
should be no charge.

Edit: Your mileage is not a lot, but how often you ride in the rain and/or snow
slush will have an affect on wear.