If you can't find a couple of wires in the wrong spot, then I think BigBird nailed it. I did this once when I was in to much of a hurry and the engine acted exactly like you discribe. Ended up that I had lined up the timing mark on the cam sprocket with one of the dimples on the crank sproket, not the timing mark. I was very lucky not to bend any valves and after I fixed the problem the engine worked perfectly.

Jack