Hi Robert,
Welcome to the forum.
I tried to find the service manual for the 20 series Tanberg's but couldn't find it.
IIRC, that machine uses a belt for the capstan drive. If that's the case, that's the first place I'd look.
If you haven't used the machine for a while, those belts tend to harden in the shape it was stored at. And since it loops around 2 ends, you end up with a belt that is oval shaped and will cause an audible speed changes as it thumps it way around.
Some belts turn to goo with age and these obviously need to be replaced too.
It could also be caused by gunk in the tape path. Carefully cleaning every surface that the tape contacts is important to keep the tape speed consistent. Take a good light and magnifier and clean your tape path paying close attention to the inside corners of all the guides. If there are any rotating surfaces, be sure that those are spinning freely.
Hope this helps.