Hi, what GPU to you have?

Have you check your system BIOS to see if battery is set to "Hybrid"?

This stupid function enables the laptop to draw power from the battery when the laptop needs more power than the AC can provide - Either a design flaw or just being cheap with parts, not sure which.

Hybrid charge will seriously reduce your battery life.

Try disabling hybrid battery behaviour in system BIOs and see if that helps, if it does you may need to replace your AC with a large capacity AC.

But you may also have other problems, some of the symptoms you describe suggest something else