windows 8 incompatibility... the 7870 should be giving you 90 fps max setting x4 aa.
the o.s doesnt like games. and will likely not like em for at least another 6 months till m.s patch windows 8 with a service pack.
best you upgrade to windows 7. there is no reason to run win 8 for gamers. it brings nothing but issues just now.
yes i say upgrade because windows 8 is a backwards step as far as my personal assessment of windows o.s goes.