Imac G3 screen taring on top and bottom

I just bought a imac g3 snow summer 2001 that appears to have been dropped, well I started it up and noticed the screen vibrating so i changed the resolution from 1024 to 800x600 and it fixed it for a few minutes then the top and bottom of the screen started to do what i have seen monitors do when they cant handle a refresh rate like split the cursor in two on every other line. well i opened it up and it looked fine and then i noticed all these tabs on the back of the tube and i was wondering if these could have gotten knocked out of alignment. any ideas? I think its running osx 10.4

CRT monitors like the one in your iMac G3 just tend to wear out after so many years. Make sure there are no vibrations near your imac, or any air currents such as from a fan. You also might be able to improve it by changing your Hertz to 60.

The tabs on the back of the tube control the focus. You should of made sure the connection on the back of the tube was tight. Just dont touch the red high voltage lead that plugs into the side of the tube. Theres two little screws on a box up on the side that change how the picture fills the screen. I would reinstall the OS and see if the video image gets better. The firmware may not be right on the machine.