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.
I have used I19 mostly and have 1-2%. 2x 7950s 680s-700s kh/s ea.
After tweaking for a few hours a day for a month approx. and documenting every little change and result I found that doing it myself gave better results than anyone else's that had ever been posted. Follow the basic setup guide as written in the scryptreadme and take your time. Read it. Read it again each time so you know what you are changing and see the results for a couple minutes with each change before saving a new conf file and tweaking some more. Keep a decent backup of your .conf file
so you can copy paste into new startups or new .conf files etc.
Set port for LTC for new setups so you can pretty much be consistent and tweak it 'til you get maximum hashrate+0 HW errors+comparable WU rate+low stales/rejects+80C or less temp+rig never crashes for any reason+you can live with noise/elect bill etc

GL!