Intellisense stops working for me in VS2012 in the following scenario:
1. In a C# code window, I type something that will bring up Intellisense (for example: "this.").
2. With Intellisense showing, I click on a different document tab to go to another code window.
3. I click back on the original code window. The cursor is located just after the period in "this." and Intellisense is not displayed.
4. I hit backspace and re-type the period.
At this point, Intellisense stops working on that code document. I can sometimes bring Intellisense back by closing and re-opening the document. Otherwise I need to restart VS2012.