Hi nonac!
Short answer: Yes could be a shutter problem. I'd ask Canon service about their opinion.
But before that maybe look at possible other factors.
You say that this only appears sporadically and only in portrait orientation?
With 1DX and grip this would be the top of shutter/pic in landscape orientation, but the bottom of the shutter as the projection on sensor is turned arround, correct?
==> So never at the top in landscape orientation. Hmm... maybe gravity helps the shutter here...
Some other aspects:
- Does it appear only with the big and heavy lenses like the 400/2.8? Or with smaller ones, too? ==> Mount problem?
- Does it appear with specific CF cards or with any card? ==> SW problem?
I really hope, you'll find a solution for this.