Sounds like a hardware performance issue to me. What are your system specs? Have you set the graphics settings to the lowest possible? Also, have you tried forcing the game to lower the settings via the commandline.txt method? If not, try it out. The commandline options are good even for me when I'm running it on a high-end PC, as it actually optimizes the game better than just using the graphics settings in the game's menu. Make a new text file called "commandline.txt" in your game's root directory and copy/paste the following lines in it exactly as shown:
-novblank
-norestrictions
-safemode
-frameLimit: 0
-availablevidmem: 2.0
-percentvidmem: 100
-minspecaudio
-noprecache
-nomemrestrict