I am currently having the same issue as you. While I have not found my personal solution yet, I can offer a potential solution.
While everyone who has been providing answers with your AC adapter, voltages, and dirty gameports (which any of those could be correct), another issue could be your capacitors. If the capacitors in the motherboard are going bad, it could be time to replace them (as when they go bad, you can get a bad image and bad or static for sound). I intend to do this with my genesis even if it is not my solution, as a way to increase its lifespan. I would recommend you look into this solution as well. YouTube is where I learned about this.
If you find you want to do this method, just make sure you get the correct version of capacitors for your specific genesis. I have heard good things about the capacitors from Console5. Just make sure you have your soldering equipment as well!
Best of Luck!
* Jakes Restorations