High invalids mining alt coins

So recently I've switched from mining LTC, to mining various alt coins depending on what's most profitable, and I found that when I switched, my invalid % went up fairly dramatically compared to LTC. I'm on a VARDIFF pool, but the blocks still come pretty quick, so I figured that was the cause, but many others don't have the same issues I have.

My config is: (4) Powercolor AX7950 with 4GB RAM, Sempron 145, running Win8 64-bit. CGminer 3.7.2.

I've tried a number of different TCs, scan time, expiry, queue length, overclock, underclock, voltage, etc settings, and the only thing that seems to make any impact, is changing the intensity. If I run at I 20, I sometimes get 20% invalids! Where as on I 18, it's a more manageable 7%, but still high, and I suffer lower khash because of the lower intensity.

So my question, as a newbie, is what can I change on my rig that would enable me to get to i 20, without having a ton of invalids. Do I need more RAM? Any ideas welcomed.