Ditto for pedal at fault. Once had this happen on a bike, and I had the LBS tighten everything in sight and it wouldn't go away. Turned out to be a sliver of metal from the pedal bearing. Could also be some dirt or even a pebble stuck in there. Borrowing a different pair of pedals is a good way to verify.
While speaking of pedals, if you feel your feet are too far apart, have you considered adjusting your cleats?