Black Spots

My last roll of Ilford Delta 400(new),35mm 36 exp, has some puzzling opaque black spots on one or two frames. They almost look like the type of pinhole that is evident on Kodalith type films. Film was developed in XTOL 1:1, at 75 deg F, I use Indicator stop bath at 1/2 recommended dilution, Fixed in a hardening fixer, treated in Orbit Bath, washed for 10 min, treated with photo-flo and air dried. My question is: Could the stop bath be too strong especially for an emulsion sofetened in 75 Deg developer for several minutes, causing the pinholes. I must say that these pinholes dont look like the stop bath induced pinholes I am accostomed to seeing (I had pinhole problems before but halving the stop bath dilution fixed it for Tri X, Plus X and old Delta 400). These are perfectly round with a very sharp edge. I don't think they are air bells because they are opaque and I give the developing tank many hard raps during processing. Any suggestions?

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