I think I figured it out.

When I was slowly cycling the machine over, I noticed the tension take-up spring would suddenly snap back, as if the thread was catching somewhere. Taking the needleplate off and observing the stitching, I noticed that on the upstroke of the needle, the thread would catch where the bobbin case positioning finger holds the bobbin case. As the takeup arm would pull upward, it would suddenly release,

I figure this is where the thread was breaking at high speeds. the hook race was a bit sticky as well, which would add to the drag forces at that pinch point when the speed increases.

I pulled both the shuttle hook and positioning finger, cleaned them, and timed and positioned them as specified in the service manual. I dialed the upper tension up, and have done a couple of test stitches at full speed and haven't encountered either the intermittent looping or the thread breaking yet.

So in conclusion, I suspect the positioning finger was the cause of my troubles - the positioning finger was too close to the shuttle hook, which formed a pinch point for the thread which would cause the thread to break at high speeds.

If all else fails, you might check the needle size against the thread weight. Bernie Tobish appeared on The Quilt Show a while back and explained why you could have tension problems if you don't correctly match the needle and thread sizes.