There are three different settings for timeouts I know of... ImplicitWait, PageLoadTimeout, and ScriptTimeout.
ImplicitWait tells the browser how long to look for an element before quitting.
ScriptTimeout is how long a script should run before quitting.
PageLoadTimeout, is well, how long we should let a page load before timing out.
My guess is that the ScriptTimeout is getting exceeded. Try running the following code before your sleep statement.
Driver.Manage().Timeouts.SetScriptTimeout(new TimeSpan(0, 0, 70, 0)) ;
Driver.Manage().Timeouts.SetPageLoadTimeout(new TimeSpan(0, 0, 70, 0)) ;
If you have any background request going that might make Selenium think there is an ongoing operation (AJAX?) try setting the ImplicitWait higher also.