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#21
In the screenshot you posted of Performance tab, I noticed the "Up Time" is ~3.5 days. I've never had the misfortune of encountering anything like this, but sounds a lot like a memory leak. I'd suggest restarting (no hibernate/standby/etc) and monitoring memory usage regularly until it gets up there again. You could monitor manually, but IMO you should look for a way to automate the monitoring and log to file. This should be possible with built-in tools*, but there are probably 3rd party utilities as well.
Once you get back into the range of 80%, close programs one-by-one, wait a minute, and see if the memory usage drops. Repeat with any non-MS services.
If that doesn't bear fruit... IDK. Wikipedia suggests a memory leak can be caused by device drivers. Are all your drivers up to date? Do you use any no-name or legacy hardware with old or otherwise suspect drivers?
* If nothing else, run this in PowerShell (and leave it running until usage gets high):
Disclaimer: That's probably not the 'best practice' way of redirecting output in Powershell... I'm a Linux guy with limited PS experience.Code:Get-Counter -Continuous -SampleInterval 300 '\Memory\% Committed Bytes In Use' >>~/Documents/memory_log.txt