Your clocks are fluctuating on lower-setting modes. Try enabling Per-Pixel lighting, as to stress the core more, at ~2xIR, see if it makes a difference.
If it makes it less laggy, disable PowerPlay by:
Enabling Unofficial Overclocking (without PowerPlay support)
over/underclocking
If it makes it less laggy, disable PowerPlay by:
Enabling Unofficial Overclocking (without PowerPlay support)
over/underclocking
in a perfect world we would all be piles of sand with no ability to form coherent bodies of body