I had this very same problem a couple years ago. It started when I bought a NEW Renthal rear sprocket. After installation I found I had a tight spot in my chain. I marked the rear sprocket at the tight spot and found every time the mark came full circle, the tight spot reappeared. I removed the sprocket and replaced it with the OEM sprocket, the tight spot went away. Thinking I had a bum sprocket, I took it back to my local dealer and he gladly gave me a new replacement Renthal. Put it on, same thing, tight spot with every rotation of the rear sprocket. Put the OEM back on and no tight spots. So,,,,,,,,,, back to the dealer and got yet another Renthal replacement sprocket. Same ole thing, tight spot on every rotation of the sprocket.
Fourth time, I picked up a Vortex sprocket and "BINGO" no tight spots. I have used only Vortex sprockets since and never had another tight spot on a chain.
If you want to know if it's your sprocket, just mark the sprocket so you can see each rotation. If the sprocket is out of round or the cut of the teeth don't allow the chain to lay all the way down in the grooves, you'll see it gets tight in the same spot on the sprocket with each rotation.
If you have a Renthal Sprocket, check it closely!