Mysterious problem

Double check that the rockers aren't hitting the retainers and holding open the valves. I had that happen once with a set of crane rockers. Didn't even think to check for clearance when I installed them and when I started it up it only ran on 5 cylinders. I could just barely see when I looked close but when I backed off the hold down bolts I saw the valves come up just a little bit. That was enough. Put the stock rockers back on and it ran fine.