I have no clue actually since I had the same issue. I just live with it since 180-300fps is decent enough (after the multicore render enable). At the beggining I though that the memory caused the issue (Im rocking 2400MHz), but it isnt.
then you have heat issues that cause the CPU to throttle. The 7***k series has massive temp spike issues. you don't list a CPU cooler so id guess its a big part of the problem.
download core temp and run it while you play. whats the max temps you see?