Digital BW, The Print

I usually create an edge burn in Photoshop for my black and white images, much as I 
did in the darkroom, so that the edges fall off about 5% to hold the viewers eye on the 
image.  The way I have done that in the past was:
1-Create new layer
2-Fill the layer with black
3-Add layer mask
4-Make selection for edge using rectangular marquee or lasso (select center then 
select inverse to get the edge).
5-Backslash so I can see the overlay for the mask.
6-Use black as foreground color and paint in the edge completely with paintbrush. 
7-Deselect
8-Change layer style to "Multiply."
9-Apply guassian blur to mask. Because I am working with large files, I've applied 
250 blur, three progressive times.
10-Work with opacity of layer to adjust density of edge burn.

This worked in the past just fine, but on a recent project, I've noticed straight lines 
that look like boxes (in synch with the frame edge) within the mask. and can't figure 
out what is happening.

Any suggestions would be appreciated.