Make sure the Nvidia graphics are enabled in the BIOS and that you are not running on the i7 integrated graphics. You may need to reduce some settings and some of you issue could be related to your internet connection as well. If the troubles are mostly during massive multiplayer sessions, consider reducing to high settings if currently on Ultra, or medium if currently on high.
Make sure you have the latest GPU drivers installed using the CLEAN method, as outlined at the following link. Ignore the rest of the article and focus only on the CLEAN driver installation portion:http://www.tomshardware.com/faq/id-2492424/laymans-simple-guide-solving-graphics-card-issues.html