In Visual Studio 2010, sometimes within the same session (not having rebooted the app) intellisense gets turned off. The checkboxes in Tools > Options > Text Editory > C# > Statement Completion, “Auto List Members” and “Parameter Information” become unchecked.
I have Resharper installed as well. But Resharper does not control that setting, as far as I know.
This can happen a couple times a day to me, all without restarting Visual Studio.
There is, apparently, a documented bug for Resharper: http://blogs.jetbrains.com/dotnet/2011/02/resharper-513-is-released/
Excerpt: