I'm sorry to tell you, this is exhibiting the exact symptoms of a Subaru headgasket gone bad. As was stated, the bubbles rising are exhaust gasses being pushed from the combustion chamber into the coolant. You should pull over just when you get off the highway and take a look at the coolant bottle, you may see the bubbles there, you should also see the coolant level rise but not get drawn back into the system to the point where after a few trips it will overflow the bottle.
Here is a video from mine where you can see it bubbling violently in the coolant reservoir after a hard run.
