opienick

5 year old 120mm AIO - probably not really gonna cut it.
 
I would try any stock intel coolers laying around to see if you get better performance.

if you have multicore enhancement or an equivalent turned on in bios, I'd try turning it off first. The 9900K is also a hot chip so while a 120mm may be alright for a 9700K it is definitely getting slaughtered by a 9900K once you put any load on it.
 
I'd also check your mounting pressure - a thinner application of TIM might actually be better at showing you if anything went wrong during the mount.
 
Definitely invest in a better cooler, though.