Question Gpu frequency

[eminemmono]

I just wanna ask a simple question about my gpu. I have 1080ti, I overclocked it, but I notice something weird that is, when the gpu load reaches 99%, the frequency of the gpu core stars to be lower. I don't know why it isn't stable on one frequency during the full load.
I5 8600k overclocked
Z370 gaming 5
2x8 rams 3400mhZ

 

[CountMike]

Depends how you overclock, you may need to raise power limit, temperature limit and Voltage.

 

[eminemmono]

Thank u Mike, but actually I raised everything to the maximum