Spots on negative question

Since I use HC110 for developing TXP320 in a Patterson tank with MOD54,
I get these spots on my negatives.

I suspect they are air bubbles, but since I have never had this before, I am not sure.
The place on the negatives where I have these spots is in the bottom to the middle
of the developing tank.

Agitation is continous in first minute then 15 secs every minute. Dev time is 6min 20secs in 80ml HC110 stock solution (is 20 ml concentrate) in 1000ml of water (1 part stock + 17.5 parts water).

I hope the picture is somewhat clear. I do not have a scanner, it is a picture taken from my light table.

I have never had this with 4x5 FP4 in Rodinal in the same tank.

If it should be air bubbles, can a presoak (how long ?, agitation ?) help ?
Can it be that these problems arise with one film/developer combo and not with another combo ?

Thank you !

edit

dilution is 1 part stock + 12.5 parts water
2 "good" taps on bottom of tank after each agitation cycle

Look carefully at the film, letting the light reflect off of the surfaces. Make sure they are not droplet marks from drying, solvable with photo flo.
The scan looks like a low contrast positive, and those are clouds - if so, the spots look like they have more density than the surrounding areas, which wouldn't happen from bubbles in the developer.

The photo is from a 4x5 negative on a light box, not a positive.
(a crop from the problem area).

The spots can immediately be seen after first rince after fix. So no drying marks.