A
The problem may be caused by a hardware fault. The machine should be tested promptly before the one-year warranty expires.
Make a "Genius" appointment at an Apple Store, or go to another authorized service provider. You may have to leave the machine there for several days.
Back up all data on the internal drive(s) before you hand over your computer to anyone. There are ways to back up a computer that isn't fully functional—ask if you need guidance.
If privacy is a concern, erase the data partition(s) with the option to write zeros* (do this only if you have at least two complete, independent backups, and you know how to restore to an empty drive from any of them.) Don’t erase the recovery partition, if present.
Keeping your confidential data secure during hardware repair
Apple also recommends that you deauthorize a device in the iTunes Store before having it serviced.
*An SSD doesn't need to be zeroed.
B
Unlicensed or cracked commercial software, or any software downloaded from a torrent, can't be expected to work reliably.
C
You installed a Safari extension ("FastestTube") intended to circumvent restrictions on downloading YouTube content.
Everything on YouTube is copyrighted. The copyright holders decide whether to allow their content to be downloaded as well as streamed. Those who choose to allow downloading can place a "Download" link on the content page. It's safe to use that link. If the link isn't there, saving the content as a file for re-use is copyright infringement, as well as a breach of the YouTube user agreement.
You may not care about that. But installing any software that helps you to infringe copyright, or to do anything else illegal, is one of the most likely ways to become infected with malware. No YouTube downloader can be considered safe to use. Some may not be malicious, but you have no way of knowing which.
D
The symbol cache is very large. It stores data used in logs and crash reports, and it may have grown because it's corrupt. The cache is created automatically, and if deleted, it should be recreated when you restart the computer. Initially, at least, it will be a lot smaller.
I've seen one unverified report that a system became unbootable when the cache was deleted. That would have been due to a corrupt installation of OS X, but you should be prepared to reinstall if necessary. There is no reason to panic as long as you have good backups.
Back up all data before proceeding.
Please triple-click anywhere in the line below on this page to select it:
/System/Library/Caches/com.apple.coresymbolicationd
Right-click or control-click the highlighted line and select
Services ▹ Reveal in Finder (or just Reveal)
from the contextual menu.* A folder should open with an item named "com.apple.coresymbolicationd" selected. Move the selected item to the Trash. You may be prompted for your administrator login password. Restart the computer and empty the Trash.
*If you don't see the contextual menu item, select the line below and copy it to the Clipboard by pressing the key combination command-C:
/System/Library/Caches
In the Finder, select
Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You may not see what you pasted because a line break is included. Press return.
A folder named "Caches" should open. Inside it is a subfolder named "com.apple.coresymbolicationd". Move that subfolder to the Trash, restart, and empty.
