It could be to do with the refresh rate of your monitor (the frequency which the image on the display is updated). So, your monitor's display is currently being refreshed at a rate of 75 times a second.
You may be seeing these lines because of lighting interference. For example, if you have flourescent ceiling lighting in your office, it can cause your monitor's display to appear to be 'flickering' because office lighting also has a refresh rate (it flashes on and off many times a second, virtually undetectable by the human eye). Any lighting powered by an AC current will do this.
Try lowering the refresh rate of your CRT monitor to 50Hz, or 60Hz and see if it goes away. Also, try higher fresh rates if possible.
If this fails it's most probably interference. CRTs are susceptible interference, and it can be difficult to eliminate. Anything which has oscillating frequencies (for example speakers, other monitors, cordless phones and mobile phones around the monitor) can cause interference.
If removing all possible sources away doesn't work, i'd think about investing in an LCD/TFT monitor