Advice?

I been playing a Modpack called "Don't Eat The Stone" and ive been getting lag and my computer is a pretty good computer-
Specs-

Manufacturer: CyberPowerPC
Processor: AMD FX(tm)-8100 Eight-Core Processor 2.80 GHz
Installed Memory (RAM): 8.00 GB
Windows 8.1

Can some one help me decrease lag? Cause it goes from like 70 Fps to 10-0 then pack to 70 it really boths mean and settings are all on low, Radar is usually on short or normal, And is a 64-Bit Operating System