When you perform the crunch, concentrate on your chin going toward the ceiling. It makes it harder to concentrate on the motion of the crunch at first, but you will instantly feel the difference. I might even suggest putting your hands behind your neck for support. Putting your hands behind your head actually works as a support during a crunch, by crossing them over you do remove the temptation of pulling with your hands to complete a rep, but you also lose that support.
If this doesn't help, I think it's either a stabilizing muscle weakness or injury. If weakness, try maybe incorporating neck exercises or "nexercises" as I like to call them. Be careful when doing these of course, look up routines on YouTube or bodybuilding.com. I usually put a dumbell on my forehead (with a towel) and do neck curls this way, then DB on the back of my head, head hanging off bench once again. Then I do left side curls and right side curls. Do a slow controlled movement, only using your hands to lightly hold the DB so it doesn't fall off. If you can use a weight plate, do this instead (I only have access to DBs right now). There is a YouTube video of a guy doing a ton of reps at literally "break neck" speed. I would NEVER recommend this. Although he has built some considerable strength in his neck, he must've gotten lucky not to incur an injury up to this point due to all of the jerky motions he does.
Good luck!
