Q: imac freezes when left inactive for a time.

Is your data backed up?

Have you run the hardware test?

Hardware test instructions.

If you have more than one user account, these instructions must be carried out as an administrator.

Launch the Console 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 Console in the icon grid.

Step 1

Make sure the title of the Console window is All Messages. If it isn't, select All Messages from the SYSTEM LOG QUERIES menu on the left. If you don't see that menu, select

View ▹ Show Log List

from the menu bar.

Enter "BOOT_TIME" (without the quotes) in the search box. Note the timestamps of those log messages, which refer to the times when the system was booted. Now clear the search box and scroll back in the log to the last boot time after  you had the problem. Select the messages logged before the boot, while the system was unresponsive or was failing to shut down. Copy them to the Clipboard (command-C). Paste into a reply to this message (command-V). Please include the BOOT_TIME message at the end of the log extract.

If there are runs of repeated messages, post only one example of each. Don’t post many repetitions of the same message.

When posting a log extract, be selective. In most cases, a few dozen lines are more than enough.

Please do not indiscriminately dump thousands of lines from the log into this discussion.

Important: Some private information, such as your name, may appear in the log. Anonymize before posting.

Step 2

Still in Console, look under System Diagnostic Reports for crash or panic logs, and post the entire contents of the most recent one, if any. In the interest of privacy, I suggest you edit out the “Anonymous UUID,” a long string of letters, numbers, and dashes in the header of the report, if present (it may not be.) Please don’t post shutdownStall, spin, or hang logs — they're very long and not helpful.

This sounds like a possible kernel panic.    Generally if the hardware test fails to see anything, the RAM is to blame.  Especially if you haven't installed anything new between the before it starts misbehaving and after.

The graphics processor is failing. 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.

Print the first page of the panic report and bring it with you.

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

*An SSD doesn't need to be zeroed.

I don't know whether your backups are sufficient. If you backed up with Time Machine and got no error messages, they should be.

Vintage and obsolete products