First, you can check whether your simulation results are convergent, if it is not, you must increase the Maxium Number of Passes.
And, you can do Lambda Refinement, such as Target 0.1, and select Low-Order Soution Basis. Until you get correct result, for my experience, 1 wavelength 10segements should be enough for calculation.
If at one frequency point you can simulate, I can not figure out what kind error would happen in frequency sweep. sry cant help you.
Hope I make myself clear.