That's a CTR monitor. What you see is a fore sign that it's at the end of it's usefull life. That flashing will increase untill the monitor dies totaly.
The most probable cause is the fly-back capacitor dying. The replacement, if you can find one, will cost about the same as a new monitor.
Get a new monitor!
Electro
September 2012
I can answer this question...
Help, the contrast screen keeps flashing on my emachines eview 17f3 monitor?