I have a monitor hooked up to my ComCast cable box in the exercise room. After I exercised for a while, I would get the stupid HDCP warning and/or the video would just cut out. I switched cables, I switched HDMI - DVI adapters, I switched monitors. It seemed like every time I started exercising, the video would stop.
It got worse recently, making it easy to diagnose. It got to the point where the video went away within a second of starting the treadmill. It is an EMI issue. Either the treadmill is emitting too much, or the ComCast box's suicide circuit is too sensitive.
I am so pleased that my ComCast has a suicide circuit to protect me from evildoers who modify treadmills to steal valuable copy righted material.