SteamVR Games Render much longer time after several hours

[vicdxxx]

I had started steamvr games, then I went outside. 6 hours later, I went back and I found the game's image stuttering. I am curious and check the frame timing window of steamvr, I found the app scene render time is longer the 14ms. Then I reopened the game, and frame timing show 5ms render time average.

Then I keep the game running about another 6 hours, and the render time beyond 14ms again, the game's image stuttered again!

What does time change actually? Is it a bug of steamvr?
Any suggestion is high appreciated!

Graphics card: NVIDIA 1060
Operating system: Windows10
Version of SteamVR: Latest