Thank you for your help.
@eraste: Thank you for your insight, I thought something like that might be happening but that is a better articulation. Yes your frequency sweeping solution is what I typically do and it works for finding the S parameters at f0, but you see if the center frequency is not the resonant frequency of the cavity, then the fields I am plotting are not truly the resonant fields.
@planarMetamaterials: Seeding is not something I've tried, thanks that's sure to help with other cavity simulations. But for this application, I don't think it will help. I'm afraid I don't have the convergence data on me, but its pretty clear after about 30 passes that the max mag delta S is still around 0.05 after about 8 passes and just oscillates there. To me, that seems a high delta S to converge to, but maybe that's OK? I do not have a lot of intuition for this.