Homemade BTZS tubes, excessive 'center' density

I've been using a set of homemade BTZS tubes made from ABS pipe. They work fine, except every now and then a 4x45 negative exhibits slightly higher density in the *center*. Edge density is fine.

Anyone have any idea how this can happen?

Let me stress that the density is in the center of the negative, and is not a 'streak' running width-wise across the negative. If you look at a negative, you'll a circular section (about 2" in diameter) of slightly higher density in the center of the negative.

A friend who has a Jobo couldn't explain it either.

Ideas?

BTW: HP5+ 4x5 sheets, Ilfotec HC 1+31, 7:30.

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