I am having the same problem - I have noticed that with the blower drive belt removed, the problem goes away, or at least it seams to. (By the way, with the blower belt off, I pick up 4 to 5 mpg.) Anyway, I am leaning towards the cat-converter also. I hate to buy one if that's not the problem as it will run right around $200 just for the part. Has anyone replaced their converter and resolved this problem?