What about your CPU % when only mining with multiples GPU (for Litecoin as an example)?
Is your computer also usable for daily uses (not games indeed!)?
Although little CPU is used it can still make using the PC cumbersome but it seems to be motherboard and card specific. I have one that mines scrypt but I have to keep the intensity of the primary video card down to 13 to stop stuttering occurring when moving a window about. Another that has no problem until the I gets to 18.