hi, when you put the bike on a stand and tighten down on the preload cap does it result in the same problem? Look at the area between the head tube and the conical spacer.

I built up a P3C, had a similar problem and the problem was that the FSA conical spacer was actually in contact with the head tube surface resulting in the locking up. The problem was that the split ring race for the upper bearing was too thin, hence the conical spacer was not high enough. You could try using another race that is thicker.