Troubleshooting for an arcade millipede game

Hello,
I bought a standup Millipede Arcade game 2 weeks ago. The previous owner didn't use it very much at all. It's been working completely great for a total of 5 hours over that 2 weeks. It just developed a wave on the screen that slowly moves the entire length of the screen. The graphics are still perfectly crisp and the game still plays perfectly fine, but it has a wave in the picture. To be honest, it's really trippy and slightly cool, but I would like it to remain normal. Do you have any advice as to why this is happening? Thank You.