It sounds like your monitor but could also be your graphics card. Simplest test is, if possible, to check the monitor on another machine. If it shows the same problem this will confirm the monitor. If not, its your card.
The symptoms you describe sound like loose connecter on graphics card out (re-do connection while unit is off( check for bent poorly fitting pins))or monitor colour guns turning off which could simply be a dry joint /defective transistor/ capacitor.. A visit to repair shop would be cheap enough..However if another monitor is available try that first..On the other hand if your planning upgrading, Some 17" CRT monitors are less than £90 nowadays..