Weird monitor flickering only during games

[silverforte]

For the last couple of months, my monitor has begun flickering. It's an old CRT from a dell Dimension 2400, I think the model of the monitor is e773c. The flickering could be described as horizontal clear distortion that occurs in multiple places from bottom to top of the screen and only happens during gameplay. It does not happen any other time. The monitor also vibrates when it flickers and makes a slight humming sound. Monitor going bad or power supply?

 

[silverforte]

Here's an update. I ran guild wars max settings, mid range, toggled them all and nothing changed. Then I put the frame limiter on 30 frames per second. The distortion is almost gone. Could it be my old CRT is going bad and just can't handle more than so many frames?