My vote is on an air leak at the manifold/head junction. Its is quite often that the manifold end castings bind in the center section if the #1 & 2 casting is not able to move and allow the #3 & 4 casting to seat in the center section without binding.
[ ] - Loosen the #3 & 4 manifold at the head. Then loosen the
opposite manifold as well as both rubber boots. Now with everything loose, align the manifolds properly and make sure the end pieces are seated on the heads and not binding on the center section. Then tighten everything down.
I bet that your leak problem will be resolved.
All advice comes with a money-back guarantee. Good luck!