I'm guessing these are flat tappet lifters (hydraulic)? You would get the same thing if a cam lobe is going flat or other damage to the lifter... check your oil and filter for steel shavings. I had a solid roller lifter go on me... only warning was that I had to keep relashing the same valve... I didn't figure it out until it was too late Check the stud first because it's easy then make sure you are getting the same max lift on that valve compared to the others.

Good luck

-Chris