Speed drop after 3 minutes.

[RalphGreen]

Hi all.
Running hashcat 3.5.0 on my GTX 980Ti

hashcat64.exe -a 0 -w 3 -m 400 --status --remove --gpu-temp-disable 

For first 3 minutes I'm having speed 21000 kH/s but after 3 minutes it will drop to 5800 kH/s. Why is it dropping speed?

Thanks.