My ST2 used to do this. I worked out that what was happening was that the throttle butterflys were not perfectly syncronised so fuel was pooling on the back of one of them becasue the injection system thought they were still a little bit open when infact one of then was shut and one was very slightly open.
The cure was to reset the tps and balance the throttle bodies.
I don't know if i came to the right answer but my adjustements certainly cured the problem.