A careful analysis of the Tur's calendar chart will reveal that the only time the months alternate throughout the entirety of the year (with the exception of Adar Aleph) is if Rosh HaShanah is a Tuesday, or if Rosh HaShanah is a Thursday in a non-leap year. In the remaining eleven year types, either Cheshvan will be 30 days or Kislev will be 29. Why is this?
I am aware that Rosh HaShanah can only be a Tuesday if the molad fell out between Chatzos HaYom on Monday and Chatzos HaYom on Tuesday (molad zakein), or the rare fourth dechiya, and that Rosh HaShanah can only be a Thursday if the molad fell out between Chatzos on Tuesday and Chatzos on Thursday (molad zakein and lo adu rosh). I'm also aware that the fluctuating lengths of Cheshvan and Kislev are in order to keep the Arba Dechiyos in check. (Source: Shvili d'Rakia, sections יא-יב.)