Uhhh. I managed to pass the mission and get ahead in the game and it was fine again, now it's just happening again randomly or doesn't load up missions.
It just freezes (except for audio) I press ESC, a pop-up comes up telling me it has to close and I looked at the program details and it says "APPCRASH" at the top. I haven't activated any cheats again..
I really dunno what's up with it. My system requirements are fine and it is a retail CD-ROM version that I bought off Amazon, not a dodgy download or anything. No mods or any crap like that.