Quote - I load the second loop, and the IK on the limbs goes haywire and there's a lot of jittering.

I would always recommend turning IK off before saving any pose, animated or not. In my experience, saving a pose whilst IK is turned on is asking for trouble.

I don't think that IK is the only problem, it sounds like something else is going on as well. If the pose is for one of the recent DAZ figures V4, M4, or similar, I suspect that the 'bodyMorphs' actor could be a source of trouble. If your pose does not need to express FBMs, I suggest you try deleting the 'bodyMorphs' actor from the pose file. If it does need to express FBMs, I suggest that you set the 'bodyMorphs' actor to use Linear interpolation, and delete all but the first frame for that actor. I can't guarantee that any of this will solve your problem, but those are the first things I would try.