Apologies in advance. Gets Popcorn.

Someone will tell you the right answer later.... whilst you are led around the houses until someone points you in the right direction.

I assume from your description that the new free hub is free wheeling freely on the stand without the chain. However when you put the chain on and go for a ride then whilst pedalling you experience 'catching'... is the 'catching' still there when you are free wheeling? Then when you have the bike on the stand and spin the wheel, the hub should be free wheeling, you still get what you perceive to be catching.

Perhaps you need to look beyond the new free hub itself and to the wheel. Hub bearings. Take the wheel off and spin the wheel in your hands. Is it smooth or does it occasionally grind? Is the wheel true... perhaps it is hitting the brakes but that would, hopefully, be and audible thing. Stiff link in the chain... and so on.