I dont know if a six inch line would cause it to do that.........the -4 line from the n20 noid to the jet on my kit is three feet..I set it up that way so the nitrous would not hit hard.....it comes on realy smooth and I have never had a problem like that....

it does not sound like a fuel problem.....it does however sound like you have some N20 making its way past the noid somehow....maybe its a bunk noid....