IMO, On a knee and scooting backwards as a defense is stalling and it should be called early like on the first or second time the wrestler does this. Refs often are too forgiving on this especially in the first period. I believe wrestling from from your knee is not stalling if this technique is followed up by an attack and/or use to a tie-up. Adopting this technique in the 3rd period is suspect to just being a stall.
Also, hanging on a opponents head without ever legit attempts to improve the position IMO is also stalling. In this instance, I think a ref might be allow this slightly longer since there is engagement to get into this position and a ref may offer a stalemate or two but should move towards stalling if this is just to eat up the clock.
I am seeing stalling calls (like many other people out there) as inconsistent. Big10 refs seem to most consistent.