It sounds like a loose connection, possibly a defective cable.
Troubleshooting:
1) swap cables (HDMI?). Perhaps you can borrow one.
2) Try a different video input on the TV. (Don't just try HDMI, also try S-Video or Component if possible to get a clear idea of what works and what does not).
You seem to suggest you also have issues when watching regular TV. If so and that's on a different input with a different cable, your issue may be a defective TV.