Are you CERTAIN that they are seated and sealing near the intake runners? Common problem with aftermarket intake manifolds. I've had to file a relief in cast covers many times, some more than others.
They can be hitting the runner (even just a little) and appear to be seated, but aren't . A tiny gap is all takes.
I've also heard of double gasketing with cork, but haven't had to do this myself.
Of course, prep is everything. Just shedding light on the not-so-obvious.
