I am getting this also. I never noticed it before. The lag is about every 2-3 seconds and is starting to ruin the experience.
The general game is about twice as smooth as prior to the patch. So FPS is up from 30 to around 60, which is great. But this monthly lag (day 3 to 4, every time) is pretty terrible. I started getting this around year 1910 (introduction of cars perhaps?).
Please can we have some advice on how to mitigate this?