Try reducing your refresh rate to something like 70 or 75 hz.
Also, because connections tend to deteriorate over time...unplug the data cable from the computer and clean the pins in the connector... dip a toothbrush in rubbing alcohol and sweep it several times across the pins, also do the same with the connector on your video card... let it dry, reconnect and see if that helps.
As monitors age, parts deteriorate and sometimes the higher refresh rates can become unstable. You are probably getting near the time to buy a new monitor...
--->It ain't better if it don't work<---