I had the same problem, same symptoms. Interestingly enough, the same fleece roller. I struggled with it for some time trying to figure the issue out. I took the valve apart a dozen times just like you did. I ran a snake down the drains, I was able to blow water and air through the lines. Nothing helped. Finally I decided I had to tear the entire run out. What I found was a snail had made it into the line. In some strange vudu-snail-magic whenever the drain had enough pressure from flow it would push the shell against a barb fitting and block the flow. When the flow stopped it would fall out of the way.
