Originally Posted by bombusben
I would guess the chainring is out of round. Most are, though some much more than others.
This was my guess too. So I took off the chainring and rotated it 2 or 3 bolts so theoretically the tight and loose spots would be move. But the chain still seemed to be loosest in the same spot - when the left crank was between 12 and 3. Further testing was cut short by my lack of a chainring bolt spanner. When I get around to getting one I'll run further experiments.