Here I am responding to this topic 4 years later!
I had the same problem that the original poster had, step stuttering included. I was not at the correct stage with the Yvette romance when I entered Spellhold. I also had YvetteRomanceActive = 3; I have no idea why based on my search for that variable in Near Infinity.
I changed the globals to the suggested values above and everything worked.
I'm wondering if part of the issue with this romance is that it assumes that you are much farther along in the romance than I was by the time you enter Spellhold. I basically started the romance during Chapter 3. It being 4 years later, it's probably too much to ask for an updated script to account for late bloomers, if you will. Maybe an accelerated romance path if you haven't reached a certain stage by the time Chapter 4 starts? Although that wouldn't address why the YvetteRomanceActive variable was set to 3.