I don't know about Wojciech, but I have just upgraded from 2015 to 2017 and I'm having what I think is the same problem, though I'd describe it slightly differently.

What happens is not that the cursor goes directly to a random segment: first it goes (correctly) to the next untranslated segment, then it jumps back to what appears to be a random previous segment (not the previously edited one; usually 2-5 segments before the one it went to initially). At first I got the impression that it was after completing the automatic concordance search, but it still happens with the concordance search disabled. Sometimes it happens almost immediately, and sometimes after a couple-few seconds.

This is extremely annoying of course... often it jumps after I've already started typing, and the result is a mess. Any suggestions gratefully received!