Two issues with "over stabilising"
If you have a 1:7 twist and shoot 40gr rounds there is a known problem where the round can mechanically fail within a short distance of leaving the barrel.
The round is spinning so fast it effectively disintegrates.
The other potential issue with high spin rates in smaller rounds is due to increased precession. This is where the round is not only rotating around it's own axis but also moving around the axis itself. In effect the tip is "waggling" or describing small circles as it moves forward.
If you have ever seen a gyroscope in action you will have seen this.
Spin the gyrosocope up and then stand it up on a mount or rod. As the gyroscope spins the top will start to slowly spin in small circles.
Picture is better than a thousand words....
http://www.nennstiel-ruprecht.de/bullfly/fig15.htm
This is most apparent in rounds longer than they around wide
The two links show a 9mm supersonic round and a .308 supersonic round
The .308 is noticeably yawing more at that moment
http://www.nennstiel-ruprecht.de/bullfly/fig2.htm
http://www.nennstiel-ruprecht.de/bullfly/fig3.htm
In many cases though you will find that a significant factor of groups opening up can be laid at the feet of inconsistent factory loads.
Take a hundred of your rounds and weigh them, you may be supprised how much variance in weight there can be. A difference of a half a grain of powder can vary pressure by 4-5%