CPU frequency stuck at minimum speed

My Trisquel 8 system, with the 4.15.0-72-generic image of Linux (that of the "linux-image-generic-hwe-8.0" package), runs on top of an Intel i5-7400 processor: https://ark.intel.com/content/www/us/en/ark/products/97147/intel-core-i5-7400-processor-6m-cache-up-to-3-50-ghz.html

cpufreq sometimes works. Nevertheless, it sometimes does not, even right after a reboot: all four cores run at 800 MHz, the minimum hardware-defined speed, although I have a process that has been taking 100% of one of the core for several minutes. Even weirder, the command 'for i in 0 1 2 3; do sudo cpufreq-set -c $i -d 3GHz; done', to set a minimum frequency of 3 GHz for all four cores, seems to work, because 'cpufreq-info' then tells "current policy: frequency should be within 3.00 GHz and 3.50 GHz" ("3.00 GHz" has replaced "800 MHz"), but does not because the "current CPU frequency" remains at "800 MHz" (and my CPU-intensive task completes in more than four times the time it sometimes takes), a now "forbidden" frequency ("current policy: frequency should be within 3.00 GHz and 3.50 GHz") that 'cpufreq-info' reports anyway:
$ cpufreq-info
cpufrequtils 008: cpufreq-info (C) Dominik Brodowski 2004-2009
Report errors and bugs to name at domain, please.
analyzing CPU 0:
driver: intel_pstate
CPUs which run at the same hardware frequency: 0
CPUs which need to have their frequency coordinated by software: 0
maximum transition latency: 4294.55 ms.
hardware limits: 800 MHz - 3.50 GHz
available cpufreq governors: performance, powersave
current policy: frequency should be within 3.00 GHz and 3.50 GHz.
The governor "powersave" may decide which speed to use
within this range.
current CPU frequency is 800 MHz.
analyzing CPU 1:
driver: intel_pstate
CPUs which run at the same hardware frequency: 1
CPUs which need to have their frequency coordinated by software: 1
maximum transition latency: 4294.55 ms.
hardware limits: 800 MHz - 3.50 GHz
available cpufreq governors: performance, powersave
current policy: frequency should be within 3.00 GHz and 3.50 GHz.
The governor "powersave" may decide which speed to use
within this range.
current CPU frequency is 800 MHz.
analyzing CPU 2:
driver: intel_pstate
CPUs which run at the same hardware frequency: 2
CPUs which need to have their frequency coordinated by software: 2
maximum transition latency: 4294.55 ms.
hardware limits: 800 MHz - 3.50 GHz
available cpufreq governors: performance, powersave
current policy: frequency should be within 3.00 GHz and 3.50 GHz.
The governor "powersave" may decide which speed to use
within this range.
current CPU frequency is 800 MHz.
analyzing CPU 3:
driver: intel_pstate
CPUs which run at the same hardware frequency: 3
CPUs which need to have their frequency coordinated by software: 3
maximum transition latency: 4294.55 ms.
hardware limits: 800 MHz - 3.50 GHz
available cpufreq governors: performance, powersave
current policy: frequency should be within 3.00 GHz and 3.50 GHz.
The governor "powersave" may decide which speed to use
within this range.
current CPU frequency is 800 MHz.

I have not found any relevant setting in the BIOS. Has somebody here any idea to fix or bypass that problem?