Firstly does the monitor have a captive VGA cable?
If not you could try a different VGA cable.
Other thing to try, Make sure the VGA cable is firmly attached to the graphics card and that the graphics card is firmly seated on the motherboard.
If after trying all this and the fault is still there could you borrow a monitor from someone to see if this fixes the problem?
If the problem is still there, it looks like a problem with your graphics card. If the problem has gone I would suspect the CRT monitor.