I have already discarded the light-leak theory because the effect is so uniform for the length of the film, and only along the edges. Also, this problem is on my Plus-X portrait images, not the 3200 EI stuff.(posted 9130 days ago)But John Hicks's latest response rings a bell. I recall reading about a tecnique many years ago when I was in school. The author recommended getting the plastic tip from a Tiparillo cigar and wedging it between the top reel and the side of the developing tanks so that the reels can't slide up and down during agitaion. That may be my solution!
I can't beleive I'm the only one to suffer this problem though. I use very conventional techniques - agitate for the first 30 sec, then invert tank twice every 30 sec. Maybe I'm the only one shooting portraits against gray seamless in 35mm!
Thanks again to John Hicks and all who responded!