I've been scanning many of my 4x5 C41 negatives.  Though I have what I am
fairly confident are correct profiles for my scanner (an Epson 4990) and
for the films I've used for the past decade or so, I find that some not
insignificant fraction of my negatives have _very_ strange color casts
when scanned, which can be quite difficult to correct in postprocessing.

In fact, looking at the color histogram for these negatives in Photoshop
shows a huge hole in the cyan channel -- which makes me suspect that much
of the cyan dye in the film either originally developed as, or converted
later to (is this possible?) the colorless leuco-cyan form.  I am not
tremendously surprised by this since I know about 50% of my film over the
years was developed with 2-part C41 kits, with combined bleach-fixes that
are notorious for leuco cyan problems (as I didn't know at the time).

If I can reprocess the film to give normal cyan dye, I want to.  But some
people have advised me that it should be possible to repeat only the fix
step; thinking about the problem (that the cyan dye is in the wrong
oxidation state) I think it may actually be necessary to repeat the bleach
step as well.  I have also seen recommendations to use a bleach for this
kind of reprocessing that differs from the standard C41 bleach.

Can anyone offer informed advice on this?