I've been running into this issue for a very long time. The issue is basically (as stated in the title) that I'm getting a lag spike that completely freezes minecraft (0 fps) every couple seconds or so. I'm running a GTX 980 TI, 8 GB ram, and an Intel i7 4790. The mods I'm using are Pixelmon 4.2.5, PixelExtras 2.1.2, Evs and Ivs 4.1.0, Voxel Map 1.6.10, and optifine Ultra H5 for 1.8.9. However, the lag spike has occurred on vanilla worlds or worlds with other mods. The spike makes the game basically unplayable.
I couldn't manage to get footage of the spike, but I have a screenshot of the lag-heartbeat-thing: