I found the same leak on mine last night. I had replaced both TB baskets. The new one between the TB and manifold that I got from Mitsu is symetrical so you couldn't put it on wrong. Well it didn't seal. Dug the old one out of the garbage (which has the tab like you describe) put it on and all is well. So much for being a good boy and replacing gaskets. :headache:

That is what I thought. Mine are symetrical also. I bought them at Mitsu. How about using some RTV?

Try just straight RTV without a gasket. Gasket + RTV sometimes doesn't seal as well as just RTV.