Aside from airflow...
That water block is proper old school. Larkool did kits in like... 2013? Nothing wrong with it, but I don't think they had 9900k level TDPs in mind when that was designed.
If the spike in temps are very rapid it might be the blocks inability to transfer heat away from the chip fast enough. There's no way for me to state it as fact, but I'm pretty sure you'll be better off with a modern AIO like the H115. Especially if you consider that you already have the rad outside the case which really is best case scenario.
@adamr Has a H115 on his 9900k with 5Ghz all core and pretty good temps. Maybe he can weigh in.
Edit:
So quick edit, after some pondering.
If you are getting 90-100C @ 1.34V /5.1Ghz with an AVX instruction workload 100% utilization, then it's actually not
that bad considering the cooling solution.
Even though it's a "custom loop" it's comparable to an AIO in it's current configuration with a slightly bigger thermal mass in the form of a reservoir.
5.1Ghz or more specifically 1.34v on the 9900k is really larger loop territory. So as I see it you can do the following;
1. If you have confidence in the cpu block you can add another rad.
2. You can get a something like a 115 which has a 280mm rad giving you 36% more radiator surface. You lose the thermal mass/res so water will heat up faster but also dissipate heat quicker. You also get a better CPU block. It should be fine for gaming, but full load 1.34v will still overwhelm a H115. (unless you're a Reddit user in which case your H100i can handle 1.8v on a 9900k no problem).
3. See how low you can go on voltage @ 5Ghz. Most mid range chips do 5Ghz @ 1.29V - 1.31V easily.
Option 3 doesn't cost anything, will drop temps significantly and the performance cost outside and even in benchmarks is negligible.