Also, any interferance with the GPU except the miner can cause GPU to go idle/low power state.
F.e. - Screen saver/GPU-Z/MSI Afterburner/any soft which uses GPU.
Some software on launch/some rendering/profile apply/etc makes GPU to enter 3D mode, after it is finished GPU goes idle/low state.

Or it simply hangs in OpenCL due to unstable clocks and then resets itself.
Visually it can be seen on display as very quick flicker.