1250 Spartan6 FPGA, I was on P2Pool since January, I had to leave it (sadly) because of Pyramining's members pressure (at that time there was a very low BTC generation compared to expectations). Since I like p2pool concept, I decided to give it another chance. Let's see if it's working better now! :-)
Are you setting a higher share difficulty for your workers? You could probably get away with 20,000-25,000 difficulty shares no problem.
You should increase difficulty with the square root of the increase in hashrate.
At 20000 difficulty, a 120 Ghps miner would see a daily standard deviation in daily reward of 10% of the mean, and the 95% confidence interval would be about +/- 18%.
At 1300 difficulty, the standard deviation in daily pay is 3% of the mean and the 95% CI is about +/- 5% of mean.
Just something to keep in mind as you add hashrate to your own setups - don't select a linear change in difficulty when you add hashrate to your set up.
Question to p2Pool miners - how many shares per day or per 10 seconds seems to be best? If you submit to large a number, how much does DOA and stale increase by?