all other machines both Windows and Linux flavors work fine with this same settings.
For some reason this particular PC never starts projects with "Suspend when computer in use" is checked.
No matter what I set the "Suspend when non-Boinc CPU usage is above %."

Machine is:
Processor Pentium(R) Dual-Core CPU E5400 @ 2.70GHz
Memory 8152MB (2742MB used)
Machine Type Desktop
Operating System Linux Mint 20 x64
NVIDIA Corporation GT218 [GeForce 8400 GS Rev. 3] (rev a2)
8GB Ram
Samsung SSD 840


OpenGL
Vendor NVIDIA Corporation
Renderer GeForce 8400GS/PCIe/SSE2
Version 3.3.0 NVIDIA 340.108
Direct Rendering Yes

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Win11 x64-Asus AMD 2.7GHzX4-16GB-GT730 2GB
Garuda x64-ASUS-Intel 2.7GHzX4-8GB-GT450
Mint21.3 x64-Xeon 2.4GHzX4-32GB-nVidia GT730 2GB
Garuda Linux x64-Intel-2.1GHz-8GB
OMV-7 x64-NAS-i3-3.6GHz-32GB-29TB

For some reason this particular PC never starts projects with "Suspend when computer in use" is checked.
No matter what I set the "Suspend when non-Boinc CPU usage is above %."

What do you expect that to do then?
What does your Activity menu say?
Is anything on that computer keeping your computer busy, think mouse, keyboard, joystick, any other peripherals that may have minuscule movement and therefore are seen as 'in use'.

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Normally any of my other machines will start running after no use, as in mouse movement, keyboard or any activity that used more that 25% cpu. Have tried moving the CPU use to 50% but still nothing.
I have Conky installed so I can see anything that starts using HD, CPU, ..It's always just a couple percent at idle and never goes above 10% unless I do something on it.
Basically same set up as all other machines running Boinc on Linux mint distro's, only this one will not start working on anything project unless I un check "Suspend when computer in use".
One other quirk with this one is Boinc is always sending "Message from server: NVIDIA GPU: Upgrade to the latest driver to process tasks using your computer's GPU"
The GPU is a nVidia 8400GS rev3, kernal: nvidiafb,nouveau,nvidia with driver 3.3.0 nvidia 340.108 which is the latest I can find anywhere for this older legacy GPU.
As far as mouse movements and such, this box is on a KVM switch with those other machines and is sitting there doing nothing, no background tasks, no servers nothing.
No other ones do this, they all start a task at the time set after is set to.

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Win11 x64-Asus AMD 2.7GHzX4-16GB-GT730 2GB
Garuda x64-ASUS-Intel 2.7GHzX4-8GB-GT450
Mint21.3 x64-Xeon 2.4GHzX4-32GB-nVidia GT730 2GB
Garuda Linux x64-Intel-2.1GHz-8GB
OMV-7 x64-NAS-i3-3.6GHz-32GB-29TB