I'd suggest you run the CPU mining with -t 3, to use just 3 cores, leaving one free for GPU miner duties (relevant if that CPU coin miner is already optimized for AES-NI). In any case, you can play with the number of threads for CPU mining all the way up to 7, just not the default (which uses all 8 threads).

I usually go into the task manager details and set the affinity of the GPU miner to be on my 1st core/thread (or the 1st two), and set the affinity of the CPU miner to the other ones, thus isolating one from the other.

(if you're not using windows, please ignore  )
~ Myagui