How are you getting 10%? 1667/(142216+1667) = 0.0116 or about 1.16% stales, not 10%.
Edit: Oh, you're talking about SS:. What do you have scantime set to? Try setting it to 30. Also try upgrading to 3.3.1.
Also I see you're using I:20. What happens when you decrease intensity to 13?
The only problem with I set to 13 is that you are gonna cut your hashrate down by a lot depending on your card, from my testing with my 7950's it cuts it down by almost 30-35% and the stale/reject rate goes from 3-5% to 2-3%. Not worth it at all.