This very much looks like thermal throttling where CPU either 'slows' itself down or 'shuts down'. But as you said you are sure that your temperatures are normal, you can check a few things.
a) Are you using any third party software for CPU undervolting, etc.? If yes, restore default CPU and GPU clocks, voltages and disable the software temporarily for testing.
b) What windows power plan are you using while gaming? Make sure its balanced or high performance. Either way, go into Advanced Power settings >> Processor power management >> Check minimum and maximum processor state on battery and when plugged in. Set minimum processor state to 5% and maximum processor state to 100%. Also set system cooling policy to ACTIVE.
Do your tests and revert if above steps helped 🙂