Have you tried going back to the 1.6's? I had a similar problem in a different engine and the valve springs were at fault. It is possible that with the extra lift from the 1.7's the springs cant keep up. Also, changing just the rocker arms changes the geometry of the valve train. I'm no expert, but I've tried alot of different things on heads and ended up torching valve guides because of bad geometry( angle of the rocker arm to the tip of the valve )
Also, check for any vac. leaks.
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