I assume from your description that it's an internal shift mechanism with cable coming from the hub, rather than an external with a cable box that unscrews? If so, it could be that the cable isn't well wrapped on the pulley, take it off and have a look, check for any fraying, it isn't a big job but a bit daunting first time. There should also be some slack in the cable, there's no indexing that requires them to be under tension. There should be a noticeable clunk when you change gear, even more noticeable when pulling the cables with the shifter detached. I'd say there's more chance of it being cable than internals, though anything is possible.
Couple of other things to check:
The oil port drain screw shouldn't be screwed in flush, I don't know what problem it causes, but there's warnings against doing so.
There are a pair of pawl springs under the shifter cap, these can weaken with age and it's a cheap and easy replacement, though I thought they could only cause slipping in lower gears. You can see them at 1:12 on this vid
https://www.youtube.com/watch?v=WGNFZjavXcIRohloff's advice now is to use 15ml of oil, I can't see how sticking with the original 25ml recommendation does any harm, but 15ml leaves less to leak out.
Good luck