I had a serious problem with the game, or more likely with my system.
On every start of an ingame month the game lagged, but it was not too bad.
However, this problem became much worse later on.
The game started to freeze on those time intervals.
It was some strange nonsence caused by the Windows Error Reporting process (Werfault.exe)
After i realized it could be a potential reason, i searched for a way to disable it.
I did it and the game is running fine now.
I don’t understand what was the problem, but it was annoying as hell.
I hope i could help with this…