I went through this with a 22RE Toyota. They don't have lifters or pushrods. one end of the rocker arm rides on the cam, the other on the valves, and the rocker arms ride on shafts. You adjust them similar to solid lifter rockers. Anyway, after a ton of adjusting, parts changes in the rocker set-up, more adjusting, I finally just bought a new cam as well, just to take it out of the equation. The machine shop, the Toyota dealership, and anyone who would listen had no idea what it could be. We started speculating about wrist pins and I don't even remember what-all, and I finally gave up and pulled the motor. As soon as I pulled the head it was apparent immediately what the problem was. Piston to head contact. I had spun the engine over several times to check for any interference, and it was fine. What happened was that with no load, and slowly turning it, everything cleared. When it started running, the inertia of the piston coming up on the exhaust stroke was enough to take all the slack out of the bearings and allow the pistons to just barely kiss the head. Valves weren't the problem. Overdecking the block was. A dead soft copper shim solved the problem, but if I thought I could get away with murder, there would be one less (so-called) machinist in the world. Anyway, I don't know if this has any similarity to your problem, but it's probably time to pull the heads and have a look-see. | 
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