The section in the manual mentioning FMRATE and T_FMRATE applies to presets, not the mod block. The manual says that the parameters are "analogous" – this suggests that they are the same, but in this case they are not. The mod block LFO should only have two pages.
That said, you should not get crackles or crashes when using the LFO. I would suggest that you follow the instructions on UM p.53 to get the unit to a stable state.
