CPU load suddenly increase to 30-50% after 4 minutes in Windows build with 5.2/5.3.

I need your help. Someone can see it and know solution?

CPU load suddenly increases to 30-50% after 4 minutes in Windows build with 5.2/5.3 in our machine.

And it causes framerate reduction.

• using very simple sample which just show Cube without animation.

• when it start, CPU load is about 1-2%. But just after 4minutes, CPU usage increase to 30-50%. And after 15-30 minutes, it go back to 1-2% again.

• with Process explorer, there are about 10-15 Unity's thread consume 2-3% CPU for each.

• With Unity profiler, we cannot find any heavy method.

• it occurs all rendering method of DX9, DX11, OpenGL Core.

• Machine Spec. is below. OS：Windows 7, Service Pack 1 - build 7601 CPU: GenuineIntel Family 6 Model 13 Stepping 7, Processor : 32 GPU：2x GeForce GTX 670 with the latest driver RAM：32GB VRAM：4GB（2GB x2）

  And It occurs in above machine, but not in normal notebooks with Core i7 with 4 core etc.

• It doesn't occur in Unity 5.0/5.1, but it occurs in 5.2/5.3.