I'm with the valve problem too, especially with it being hydraulic, it may have too much pre-load. I don't think the timing would effect it in the way you described, one or 2 cylinders occasionally and then consistantly, timing should have created the problem in all the cylinders, not just a few. It sounds like a few of (or maybe all) of the valves had too much preload and were starting to "burn by", as it got more miles the problem would get worse and ultimately burn some valves. I don't think a cranking compression check or leakdown would have pinpointed it since the lifters pump up some under
oil pressure(one reason correct pre-load is so important) and at cranking speed wouldn't have built enough pressure to simulate the condition.