Somewhere in options, I think in performance, there is an auto-save feature. If that is set wrong, it could be that you are having the infamous "lag spike of death". If not, then you are being "minecrafted" (that's what I call it), and mc may be unplayable. My old laptop got so bad, it took 5 minutes to load. Please reply, and I hope this helps. I'm not an expert, I'm just trying to help out as best as I can.
Sometimes upon world load, and sometimes when anything big happens (Like a creeper explosion) minecraft jumps to 2k mb memory usage (max) and crashes. If someone can help, that would be great. It's not my memory, or my comp for that matter, they are top of the line. I really want to know, this seems like a REALLY fun modpack. Thanks, and I hope this helps anyone if the future.