Rendering Slot Issue

Another strange one.

I start a rendering by hitting the render button. Rendering to new window. The new window pops up, the rendering starts on Slot 1. After a few minutes, the cpus all drop and the rendering appears to have stalled. If I switch the rendering slot to Slot 2, the cpus all wake up and the rendering keeps going. But it is not the original render slot the rendering started on. If I go back to the correct render slot, Slot 1, the cpus stall again. So I have to keep it on slot 2. But you can not see the rendering progress. I can see the rendering progress on slot 1 but cpus stall out…CRAZY!!!

Blender 2.79

Anyone ever experience this?