steve,
i am like tom
the gristlepart really was the key off there and i think he is onto it.
what grade are these??
i cook select full packers every weekend and have never had a gristle problem.
one the fat part i have to ask how closely are you triming your briskets?
on the bottom is a big pocket of fat. if you cut this out it might help. your cut will be wedged shaped guess on most of mine it comes out at about 45-60 degree angle and removing that reduces the fat in your slices with no apparent lose of overall tenderness. the fat on the top i trim down to 1/8 to 1/4" overall.
hope it helps but if you got commercial grade it wont help at all on the gristle problem.
jack