I suspected that tight area might be the problem. Its probably trying to self intersect in that area.

I also tried to shell it in viacad and it came out with the cryptic message "could not offset surface as requested:Thick Solid_331", so your guess is spot on.

Thanks for the suggestion about offsetting surfaces and breaking it down into more smaller chunks. Its a little more work but should give me the desired results. Will do it that way.