As an aside, did you mean to pick that "Best Answer" above? I unselected it, you can reselect it if you wish, or select one at the end when your problem is resolved.
Now to get on with your question, how fast are your fans running at that temperature?
While the MSI Gaming version of the GTX 780 Ti is designed to run cooler than a reference model, there can be other factors that will push the temperatures up. Most notably, a hot room (it is summer after all) and poor case ventilation. The fact is that 82c is exactly the temperature that your card is designed to target, just check your Temp Limit in MSI Afterburner. Your card will spin up its fans and lower its Turbo Boost speeds to maintain that temperature in a hot environment. If your card runs well-below 82c, like it should with a MSI Twin Frozr card, then the fans will run slower and the Turbo Boost clocks will be higher.
So, there's nothing wrong with 84c, in and of itself it's exactly normal for hot summer temperatures. But the question is, how fast are your fans spinning and what speed are your clocks running? If your fans are running at 85% and your clocks are running at stock speeds, then you probably have an issue with your case ventilation. As a less likely possibility, your heatsink might not be fully seated on the GPU shroud. But first, let's take a look at your temps and clock speeds.