+1. Chances are the axle is bent . To check you really need a clock gauge, dti, but pointers fixed to the chassis and set close to the axle end and /or inboard sprocket carrier position should suffice. About 10 thou (0.25mm) max. run out would satisfy me. If you get the tight spot once per axle revolution the problem is somewhere in the axle/ sprocket carrier/ sprocket . To isolate find the tight spot and mark the rotational position of the axle against a fixed mark on the chassis. Unbolt the sprocket and rotate 180 degrees. If your marks are still aligned at the tight spot ,the sprocket is ok.Repeat for the carrier removing the key. Marks still aligned at tight spot , carrier and sprocket ok, axle bent. 25 and 30mm axles can be straightened fairly easily. Chains can get damaged causing tight spots but a new chain should avoid this.
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