Dithering will eliminate this effect. The frames are too closely matched.
My basic understanding of the problem (among other problems) is that the addition (average, median as the case may be) of pixel values during stacking increases the value of noise, where pixel x, y position is closely matched or coincident and creates this striated appearance. Calibrated frames will do this too. Light to dark frame temperature mismatch is common with DSLRs.
It is easier in the long run and more effective to space your light frames while imaging by 10 - 15 pixels. There will always be a little dark mismatch and dithering will hide this as well.
Berry and Burnell recommend dithering DSLR frames by about 12 pixels in their handbook of image processing. It works wonders. Inceases SNR (noise reduction), improves flat fielding and as mentioned masks calibration issues.
Virtually essential for DSLR's.
Added comparison image. Same camera.