A lot of ARs will leak a little bit at first but seal themselves up. Usually it's not leaking as bad as it seems. The question that was not asked is what ammo are you using? Sometimes weaker than spec ammo will cause the type of malfunctions you describe in a new, tight, or possibly under-lubricated gun.
I am thinking your gas block is not to spec. This is the first I have seen or heard of this problem. Like Robo said earlier, since it is a new upper, take it back where you bought it and have them fix it. You shouldn't have this problem, no matter what ammo you run through it.
"Democracy is two wolves and one sheep voting on what to have for dinner.
Liberty is the well-armed lamb contesting the vote." - Benjamin Franklin