El Capitan 10.11.6 freezes upon wakeup from Sleep — but Security Update already installed. How to fix?

Running El Capitan 10.11.6 on a 2008-era Aluminum iMac (7,1, 2.4 GHz intel Core 2 Duo). El Capitan already has "Security Update 2018-004" installed.

The problem: Occasionally, when waking up the iMac from Sleep, the computer/operating system will freeze -- no keyboard input or mouse movement will register in the screen. "Force Quit," either via mouse or keyboard, cannot be accessed, so I can't restart the Finder. The only way to escape the freeze is a "hard restart" by pressing the power button on the back of the computer. After the computer turns off and then back on and restarts, everything is back to normal. The computer and the operating system otherwise run smoothly with no other problems.

The freeze happens unpredictably -- I tried to isolate the cause by running or not running various applications as possibly causing the issue, but have not been able to pin down any consistent pattern, nor can I replicate the problem at will -- seems to happen randomly, regardless of what software is or isn't running, or what I do.

I tried researching the problem, and discovered that in 2016 Apple identified a glitch in 10.11.6 due to "a kernel issue that may cause your Mac to occasionally become unresponsive," in particular freezing upon waking from Sleep, and released "Security Update 2016-003 Supplemental (10.11.6)" which fixed the issue.

Thinking I had found the solution, I tried downloading that Security Update from the Apple site and installing it, only to get the error message that it cannot be installed because this computer "is not configured" to accept the update.

I then looked at the "Installations" panel in the "System Report,": and saw that this computer already has "Security Update 2018-004" installed, which supposedly incorporates the kernel-fixing code from the "Supplemental" 2016 update in it, and since 2018-004 is a later update, it is not necessary or possible to install a now-unneeded earlier Supplemental Update.

So now I'm back to Square One. Since I assumed that the freeze was caused by a kernel issue in 10.6.11 that was later fixed by the Supplemental Update, but in fact I already had installed a more recent update that incorporated the fix in the Supplemental Update, I have no way forward and still don't know what causes the problem nor how to fix it.

Any suggestions or insights would be greatly appreciated.