Workaround for a MBP 2017 overheating.

I am facing an issue with my MBP 2017 without Touchbar and without a T2 Chip.

When I am working with the MBP, for the first hours, sometimes when I perform heavy tasks with it, I notice that the FAN is activated normally to dissipate the heat. But after a few hours when not performing heavy processing activities the MBP starts to heat up to the point where the bottom part of the screen starts to show a blackish bar, which appears to be the collateral effect of overheating.

Please note that the fan works and for the last months I would never have this issue at all.

What I have tried so far:

• Restarted the computer holding the "D" button to check the hardware, and there was no hardware issue found in my MBP (on the test the fans ran for a while to check if they work).

• I tried to reset the SMC following some tutorials on the web, but apparently, on the checks which were recommended to be done upon the reset, did not prove that the reset was successfully made.

What the issue may be, as it points out to be related to a software/config issue?

As the fan works when heavy stuff is done on the MacBook I do not understand why it is heating up when navigating on the browser, for example, after a few hours. As per my understanding, the fan is activated upon heat detection.

Why does the fan work and sometimes not?

Note: I am using the MBP on a table, without bumper cases, or anything that could warm it up.