Game Stuttering after meeting the Doctor

[CostaRic]

(This is an updated repost of https://www.frictionalgames.com/forum/thread-38624.html, mods please delete this other one as I wasnt registered)

So, here is my problem. The game ran perfectly at the biggining where you are at your appartment and when you meet the doctor for the first time, but when you get teleported to the wierd underater place (I just started the game) the game starts freezing and stuttering horribly every few seconds, why is this?

My Specs are a tiny bit above minimum requirments and the game runs smoothly for the first few scenes like a I said, the only thing i can think of is my ram being an issue or the fact that my GPU is ATI, but it still is above minimum requierments.

My specs are:

Win 7 Ultimate 64 bits
Intel Core i7 1.60gHz
4Gigs Of RAM
ATI Mobility Radeon HD5800 Series
1TB of Storage