Back up all data immediately as your boot drive may be failing.
This diagnostic procedure will query the system log for messages that may indicate a hardware fault. It changes nothing, and therefore will not, in itself, solve your problem.
If you have more than one user account, these instructions must be carried out as an administrator. I've tested them only with the Safari web browser. If you use another browser, they may not work as described.
Triple-click anywhere in the line below on this page to select it:
syslog -k Sender kernel -k Message CReq 'Channel t|GPU D|I/O|n Cause: -' | tail | open -ef
Copy the selected text to the Clipboard (command-C).
Launch the Terminal application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Terminal in the icon grid.
Paste into the Terminal window (command-V).
The command may take a noticeable amount of time to run. Wait for a new line ending in a dollar sign (“$”) to appear.
A TextEdit window will open with the output of the command. Normally the command will produce no output, and the window will be empty. If the TextEdit window (not the Terminal window) has anything in it, post it — the text, please, not a screenshot. The title of the TextEdit window doesn't matter, and you don't need to post that.
