700 raidmax psu.....
can you test your pc with a corsair builder series or any other type of good psu? the raidmax should theoretically run your pc but if the psu is old or recycled from other build, it could have aged too much to support your pc.
did you do a clean reinstall of latest amd drivers? did you use driversweeper?
7850 skyrim perf on ultra @1080phttp://www.tomshardware.com/reviews/radeon-hd-7870-review-benchmark,3148-15.html
from what i've been reading, the 7850 seems quite problematic with drivers. tweak skyrim settings, especially with anti aliasing - see if that solves your problem.