Spelling Bee winnah!
Join Date: Sep 2008
Location: Rolling down Pontiac on dem dubs doe!
Even if your header gaskets are new you should keep tightening them down every few days. Remember, start at the center and go out to get a nice seal. It takes a few months before everything settles in and the header bolts / studs stay in tightly. Even then, I always check mine once a month.
I would remove the the valve covers, very easy to do, just to rule it out.
There isn't much you can mess up when installing new intake gaskets. Only thing I can think of is leaving a part, tool, towel (R3DLT1), or something in the lifter valley.