I have a situation where I local prefs file specifies says to suspend if CPU is above 60%.
When I try to use FIREFOX, I get lots of stalls and messages about long running scripts.
AVI videos are stalling and breaking up.
If I SLEEP or EXIT BOINC(SETI@HOME), FIREFOX and videos run clean.
I am on WIN 7 LENOVO T510 with 4 cores I5 processor, 2.40 ghz

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since you only have 4 cores, you realistically only have 4 load levels to choose from (25/50/75/100) even though you can type whatever number you want.

BOINC will use 100% of a thread. if you set it to 60%, it will only run on 2 cores, because to run on 3 cores needs 75%.

there's not much you can do about it other than setting BOINC to suspend when the computer is in use, or set the threshold for BOINC to suspend with CPU use to a lower value, like 25%

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Seti@Home classic workunits: 29,492 CPU time: 134,419 hours

I have an A10-6700 with 4 cores (although the Windows Task Manager says it has 2 cores and 4 logical processors) and it runs 4 BOINC CPU tasks plus one GPU task. Sometimes it runs Milkyway@home on 4 cores suspending all other CPU tasks. I have put not limit on CPU usage by BOINC and I am using Firefox and e-mail with no problem. RAM is 20 GB. It also hosts a SuSE Linux Virtual Machine with Tumbleweed OS, which is a development version with kernel 5.3.11-1.
Tullio

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BOINC will use 100% of a thread. if you set it to 60%, it will only run on 2 cores, because to run on 3 cores needs 75%.

But that's a different preference. That's "Use at most N % of the CPUs", while the OP talks about "Suspend when non-BOINC CPU usage is above N % ". The latter requires that BOINC Activity is set to "Run based on preferences". If Activity is set to Run Always, it'll run always.

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@Ernest Nachtigall

You could try using a "cc_config.xml" placed in the "S:\ProgramData\BOINC" (or wherever you installed it) with these parameters:

<cc_config>
<options>
<process_priority>0</process_priority>
<process_priority_special>1</process_priority_special>
</options>
</cc_config>

These tell the BOINC Manager to set non-GPU tasks to the bottom priority, and GPU tasks just above them. This might be enough to stop
the issues you are having even when using 100% for BOINC.

BOINC must be restarted after placing the file in the folder.

Best wishes.