I had a similar issue involving an endertank that I had in my base, so I removed the mod to get rid of it. The thing that killed my Unstable world was when I added Forbidden Magic, which had an enchant conflict with Mariculture. I wasn't using Mariculture, so I removed it.
Leaving me with a metric butt-tonne of non-updated and flowing water blocks in the ocean that my base faced. My game crashed down to single digit FPS, even with careful use of OPIS to try and alleviate lag sources. This was not useful for the times I was attacked by 5 Eldritch Guardians at once (I was doing all kinds of forbidden research to try and get to play with Thaumcraft's voidmetal toys
)
Now putting together a primarily magic-focused pack, with a bit of tech and a lot of exploration. I've taken out a few mods that I always seem to use like MFR and TE, and I'm just going to give Ars Magica 2 a few days for any update issues to be ironed out. Then it'll be ready to roll