I suck at Automaker too, I'm embarrassed to admit -- it's something I should learn more about.
However, just because I don't have an Automator answer for you doesn't mean I don't have AN answer, and that answer is: Graphic Converter.
It's designed to do batch conversions exactly like what you're looking for, and seems to handle "corrupt" or malformed JPEGs better than Photoshop.
It's free to try, so you have nothing to lose (and it's cheap to own, btw).
However, just because I don't have an Automator answer for you doesn't mean I don't have AN answer, and that answer is: Graphic Converter.
It's designed to do batch conversions exactly like what you're looking for, and seems to handle "corrupt" or malformed JPEGs better than Photoshop.
It's free to try, so you have nothing to lose (and it's cheap to own, btw).