Same happened to me, rebuilt the bike now the throttle won't return. It was never amazing, but now it just doesn't spring at all. Going to just wait until I have money and replace the grips and cables.

I suspect the angle at which the cables cross the top of the top rad is key. I bet the wrong angle causes the cable sheath to get squashed and cause drag on the cable. I changed the top rad as the old one had a broken bracket, and I don't recall if it had the plastic shield on top like the new one does. NC30 top rads don't seem to have one, I think it's because they sit very slightly lower as they have longer brackets. I have an NC30 top rad and the bottom of it ends up as near touching the cam cover on the 35 as makes no difference, and it seemed to not want to sit quite centrally either.