You’ll need to do a bit more diagnostics to help aim you to what’s needed. I would install a good thermal monitoring App like [https://www.tunabellysoftware.com/tgpro/|TG Pro] it will allow you to see more clearly the CPUs & GPUs heat levels as well as the efficiency of the heat sink and heat extraction via the fan.
So when the glitches show up what was the systems thermals? If you boost the fans RPM does that help?
You might find the heat sink it’s self has failed (the coolant had leaked out) and/or the thermal paste needs to be refreshed.
The second test is when you encounter the glitches to plug in an external display to see if the image on it is stable. As sometimes the display unit fails.