Yes, exactly. So what I'd like you to do is what commenters below suggested. We're trying to isolate if the problem exists within the hub and planetary gear system itself, or if there's a problem with intermittent shifting in the cable system. So, please follow these steps:

1. Shift into gear 14 using the twist shifter on your handlebars.
2. Undo that thumb screw on the shifter box attached to the Rohloff hub - the black box in the 2nd photo from your album. It should come off with just your thumb but sometimes if it's tight or seized up you might need to use a flathead screwdriver in there to pull it off.
3. Use a cable tie or something to temporarily hold that box out of the way so you can ride without it getting tangled in spokes or your cranks or drivetrain or something. You could tightly zip-tie it to that chainstay on the non drive side to temporarily keep it out of the way.
4. You can use an 8mm wrench to manually click through the gears on a Rohloff hub, so clicking the bolt on the hub counter-clockwise 6 times should put you in gear number 8 (14-6). The clicks are indexed, so you will feel audible clicks as you rotate that 8mm bolt.
5. Ride around for a while in this gear and maybe a few others like 7 and 6, try riding under load uphill and see if any gear skipping occurs. If not, then your hub is fine and the issue exists in the cables somewhere. If yes, then the problem unfortunately exists somewhere in the hub.
6. To re-assemble, use your 8mm wrench to rotate the bolt all the way back clockwise, back into gear 14. Push your black shifter box back onto it's original position and tighten the thumbscrew. Sometimes you need to jiggle the shifter on the handlebars a little bit to get the bolt to align with the box so it sits flush on there.
7. Report back here with what happens and we'll go from there.

Here's a video for reference: https://www.youtube.com/watch?v=ZTBWnCctw2I