Check the tips of your rocker arms. They are probably messed up. Instead of gliding across the stem of the valve as they push down, they're getting caught and pushing the valve stem to the side.
The same thing happend to me. When I assembled the head and turned it by hand, noticed that the spring was compressing a little more on one side. I didn't know any better. So i ran it and those are the valve guides that broke.
Compare the tips of your rocker arms with the broken guides with the ones that are not broken and tells us what you find.
I'm curious.