Why sudden TDP drop?

[Nebell]

I'm running +125 on GPU and +600 memory. TDP is set at 85%.
In the beginning, the GPUs are at 1950mhz with 85% TDP but quickly drop to 50% TDP and 1500mhz. The temps are not an issue, they are at about 75. The other rig with 2 more cards doesn't have the same issue.