You could have a "dirty" power supply, be suffering voltage fluctuations in the house AC current, the monitor could be dying - could be a lot of things. You can pick up an APC UPS which will give you about 15 mins backup for around $45. Another useful feature of these is that they can monitor voltage fluctuations, spikes and other anomalies that would be occuring outside your PC setup. However if you're noticing no similar fluctuations it is most likely in the monitor itself. If you select a different desktop/monitor resolution/refresh rate do the symptoms persist?