I've been having issues with a few games namely Jungle Hunt, Demons to Diamonds, Dodge 'Em and Raiders of the Lost Ark. They'll power on and the video will either intermittently go to a black screen while the sound continues, or it will go to a black screen and stay there.
I've tested them on both my 2600 Jr. and Light Sixer with the same result.
I thought it was just a case of bad carts but the same thing happened with a second copy of Demons to Diamonds.