Fogging through center of 35mm negative

I developed a roll of "New Ilford Delta 400 Professional" in a stainless steel reel and tank. The film came out with a slight ghostly fog throughout the center half of the film. I agitated every 30 seconds with 4 inversions. Chemicals were good with fresh Ilford DDX, water instead of a stop bath, and fixer that was fine. The film came out with a slight ghostly fog in the center 50% of the film throughout its length. There were occasional "lighting bolt" shaped horizontal streaks running across the fogged portion where the film was clear. Does this sound like a development problem, or possibly a shutter problem on my Nikon F5? I had this happen once before recently, but thought that it was the result of developer that I diluted at twice what I had intended. Has anyone experienced this problem before?

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