CPU throttle with clock speed spike drop
Have you set the power mode? On Lenovo, left click the battery indicator and slide it right to best performance. This should affect fan behavior.
Are you using an external monitor? If you're using the laptop monitor, try switching to an external monitor and see if it goes away.
Are you using any peripherals on the egpu (keyboard, mouse, ethernet)?
Does the issue occur in just one game or multiple games?
Have you updated firmware using vantage?
Are you sure nothing is happening in the background that could be causing a slowdown (eg windows updates)?
A couple more suggestions:
in the bios under power:
disable intel speedstep
disable CPU power management
make sure adaptive thermal management is "maximize performance" for both AC and battery.
Then this is different to a situation I had. Not sure if it will help. Have you tried downloading Intel XTU, to see if you can tweak the TDP to see if it prevents it from bursting. Do you know what your CPU temps are at the point of dropping the clock speed?