I am not a CWI and I have never welded Inconel.
With that being said, I have experienced enigma linear indications twice. We cut and bent the samples (they were test pieces so this was possible) and they both passed. It is to my understanding this can occur when welding dissimilar metals. I was also told that since Inconel welds have a tendency to crack, it is important to make sure it most definitely isn't a crack instead of an "enigma" or "ghost" line. A crack would most likely be jagged rather then a smooth blurry line.
As far as how to eliminate it, you would have to figure out what it is that is causing it. Is there any pre/post heat or a specific cleaning method? The fact that this is only happening "periodically" tells me there is something going on there and I'm sure it can be remedied.
I'm sorry I don't know very much on this specific topic but since I've seen this before I figured I'd throw my 2 cents in.