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While you have the valve cover off, lay a straight edge (or something close) across the rocker arm studs. I bet you'll find one thats pulling up on you. Each time you adjust the rocker its fine until that rocker pulls the stud up a bit more.
If this is the case, you can buy oversized studs, knock a new one in and your all set.
The other possibility is a lobe going flat on the cam which is dishing out the bottom of the lifter (causing more and more valve lash) If it's still running ok after the valve adjustment, chances are the cam/lifters are in good shape.
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