help Crashing? (different than the Boston problem... I think.)

Well, my computer runs Assassin's Creed 3 perfectly on max graphics in multiplayer, and out of the animus, and runs it well but with lowered FPS in Boston. It does work though. So I just bought a gun and a sword, and completed that mission. Now I am to follow Charles Lee inside a tavern, but when I enter, it crashes. Not instantly though. And not lower FPS than when just in Boston. It lags a little like Boston, and lets me continue sometimes until the top of the stairs, and sometimes to the person you have to walk to, but then crashes. I think it's more timed though, but that's if you just start walking instantly. The weird thing is, everyone says it's playable for them but lags, and that's (about) the same as me, except for the mission. Anyone got a fix for me?

I had CONSTANT CRASHES with this game running on Windows 7, 64-bit quad core with GTX 580 graphics card.
In BOSTON and the UNDERGROUND were by far the worst, crashing every 30 seconds (completely unplayable in parts)
The first thing i tried was to set the AFFINITY of the application to one processor, this helped to start with but makes the game FAR too slow to play and still crashed, just 90% less
Pause the game
Ctrl+Alt+Del
Start Task Manager
Processes tab
Right click AC3SP and Set Affinity...
Tick just one processor and click OK
The next thing i tried and has worked 100% so far (touch wood) is to UNDERCLOCK my graphics card
I use a program called EVGA PRECISION but you can also use the NVIDIA CONTROL PANEL or if you have a Radeon card, whatever that uses
The stock values =
GPU CLOCK 772MHZ
MEMMORY CLOCK 2004MHZ

I changed them to =
GPU CLOCK 600MHZ
MEMMORY CLOCK 1800MHZ

GOOD LUCK

Eh, it sadly didn't work. But now at least I get a message. "Display driver stopped responding, and has been remade". Just a rough translation. The PC has a ATI Radeon HD 5570, but it shouldn't really matter when I can free roam, play the game smoothly up to Boston, and play multiplayer smoothly. It's really starting to annoy me.

Well, usually this kind of stuff doesn't happen, I mean, it plays the other Assassin's Creed games, Bioshock and even GTA 4 (as long as I just set the graphics down a bit)... this really must be a crappy port