Huge fps drop/Screen freeze in nether.

Basically I can't play properly when going into the nether :S The game is freezing from time to time , cpu goes crazy from 60% to 100% from time to time. I'm playing on hard difficulty and it's really impossible to farm pigmens and ghasts not talking about fortresses ( this with full prot 4 set ). My PC is having more than the recommended requirements for MC.