I bought a new set a few years back and had same issue. The mounting flange warps after the riser pipe is welded on (my opinion). Not sure why they aren't resurfaced after welding.
A file and a honing stone were the fix for me. I complained to IMCO when ordering replacement gaskets and I believe I received a free gasket :).
I'll be buying another set this summer and will see if anything has changed. I think I'll go to the machine shop for surfacing prior to install....