I'm pushing for an air leak too. My Sport had leaking intake manifolds, and the idle would rise when it got warm, and it just continued to get worse. The intake manifolds have o-rings in them that should be replaced if you ever take them apart. DON'T be tempted to use RTV to seal the intake. That will only last a short time, as modern fuels will dissolve them slowly, and your problems will all return. O-rings are measured by the ID, then thickness. Lots of places on the web to get them.

Of course, if your intake boots are stock, they are a common cause of an airleak also. Just see if the clamps are tight, and the bolts to the manifolds are tight would be a first step.

Can you get a CO measurement to see if you are running lean on one or both cylinders?

One odd ailment that some of the ST2s had was the water temperature sensor. if it went bad, it told the computer the motor was cold and richened up the mixture. While this is opposite of what you are experiencing, its another failure point.

Cheers,
Phil

happy motoring