Skyrim sudden FPS drop during zone transition

Typically 'screen tearing' is caused by a game updating the video/graphics buffer that is rendered on the screen before the previous buffer was rendered on the screen. A basic lesson, the graphics for your screen are stored in memory and then rendered on the screen. Sometimes games will change the buffered image before the previous cycle completed. If the game pushes a frame to the buffer before the monitors refresh rate hits you will get tearing.

There are two solutions to this problem, the first comes from the fact that the update to the buffer should (typically) occur when the monitor is restarting it's rendering cycle (known as V-SYNC). If you have the option in your settings try turning on V-SYNC. If V-SYNC is on when having issues you might try turning it off instead. Generally you always want V-SYNC on unless you want tearing or unexpected graphical artifacts.

The other option is based on a typical solution that most games use to prevent the problem. They use multiple buffers (2 or 3 typically) so while one buffer is being rendered, the other is being updated. Some games have an option to enable double or triple buffering, if so, enable it.

Souce: Game Programming Algorithms and Techniques: A Platform-Agnostic Approach

So, considering the discussion in the comments, and my opinion, possible causes of the FPS drop and screen tearing for the specific game in question, CS:GO, would be (in order of likeliness)

• Corrupt configuration files on CS:GO. (OP already reset them, discarded)
• CSGO or Source Engine update issues (No updates were released at the time of the incidents)
• Busted drivers. (OP already reinstalled them so, discarded)
• Broken GPU fan, so it throttles down to prevent damage. (Op checked them, so discarded)
• Dirty GPU contats. (OP already cleaned the card, so discarded)
• Tweaking fps_max value. (Already done, discarded).
• Nvidia GeForce Experience causing issues. (OP uninstalled it, so discarded)
• Steam Overlay causing issues. (Op disabled it, so discarded)
• Windows update messing with OS. (Op stated that no updates occurred before the incidents)
• Nvidia HD Audio causing issues. (OP uninstalled it, so discarded)
• Busted Video Card (By OP's comment this turned out to be the culprit)
• Busted RAM (Unlikely)

At the time of this answer we don't have specific information about performance in other games, OP specified that the screen tearing was occurring on Borderlands: The Pre-sequel as well, but we don't have specific performance data. On the other hand Furmark stress tests didn't crash the card, but the issue in the games persist.

Given that information the only option we have left is a somehow damaged Video Card, the solution would be to fill a RMA, as the card was recently bought. I suggest contacting with the manufacturer instead of Amazon or Nvidia. The manufacturer (if professional enough) would give you a tested card, guaranteed to work, while Amazon will give you another boxed one or a refund if you are a regular client. Nvidia will do nothing they only build the chipset. I've had a reasonably good experience with EVGA for example, not sure about other manufacturers.