Do you have any buildcraft fluid pipes connected to mariculture tanks/machines? Buildcraft fluid pipes and mariculture don't always like each other. If so and if you can, when the problem starts try breaking the fluid pipes and see if the problem stops. This is a problem that I had.
I don't believe we do but I shall check tomorrow.
The admin of my server looked over my crash logs and said it looked like it was MultiMC freaking out over something specifically about our base, though what we aren't sure. If I just kill the process for MultiMC after my client is up and running, that seems to alleviate any symptoms of choppy performance.
However, since Curse Launcher has picked up Regrowth, I'm running through that and had zero problems with performance other than the standard clunky performance from my computer (only 6GB of RAM and the video card is only so-so). I run Optifine and Fastcraft as well as FPSPlus with the appropriate java arguments but I'm still lucky to pull 20-25 FPS in this modpack. I think it's just a lot of stuff going on and it slows me comp down.
Anyhoo, I consider this solved for now, but I will check on the BC pipes to Mariculture machines thing. I do have some diamond kinesis pipes running from the mariculture water turbine we built to my AE system. I suppose that could be the BC/MC thing not mixing well and will try replacing with something like universal cables or w/e instead.
Thanks!