I'm running higher Cclock and memory clock than yours and I'm not having any invalid shares, watch out for your internet connection, latency is the top reason why invalid shares occurs most time
I have two GPUs in this setup and the only one giving errors is the sapphire.
invalid or stale? is the hashrate reported on the pool significantly less than what it says in your mining software?
Invalid shares, the hashrate in the pool is the same (sometimes even more) that the mining software.