Here's an idea to try. Remove the valve cover and fire the engine up. You may want to place a sheet of cardboard left and right of the valve cover area to catch oil flicking off the rockers. With the engine running you'll see and hear if one of the rockers is causing a problem. Take your thumb and push down on the adjuster end of the rocker and you'll feel any cam/lifter/pushrod problem. It looks scary to do this with the engine running but it isn't. I diagnose flat cams this way. As you move down the eight you'll feel the one that may be the problem.
Maybe a big vacuum leak? Or a distributor cap that is crossfiring? It's also possible that the bottom of a lifter has shattered, although if you have checked the lash a few time you would have found it.
Also look inside the valve cover for any marks showing contact if you suspect that.
Rhys