Okay, an update... I did testing and noticed that this is actually the same bug as here.

My observations:

- Loading the save from Autumn (saved after the bug had first triggered), and advancing 1 turn without activating the script causes the game to jump to 14AD. I get a message that I've failed to win the game, with the option to continue playing. The script can now be activated without stuttering.

- Loading the same save and activating the script (bug triggered!) and then advancing to the next turn despite the stuttering is really weird. Not only are all the AI characters moving slowed, the year is also appearing as 14AD as the turn progresses. But when the computer factions finally finish their turns, and I get to the new turn (Winter 246 BC) the stuttering is gone, and the year is again set correctly as 246 BC! No messages about losing the game either. However, the game then displays summer graphics on the campaign map instead of winter graphics. The characters have the correct traits (winter season, and movement restrictions) though.

- Loading the save from Summer (saved before the bug had first triggered) and advancing a turn without the script, then activating the script causes stuttering.

- Loading the save from Summer and going multiple turns without script before activating it, allows to bypass the bug (script can be activated without causing stuttering). But again, the seasonal traits and the campaign map graphics ended up out of sync.

If you want my saved games, let me know and I'll e-mail them.