The older monitor may not be capable of the scan rate you had with the newer monitor. Decrease the display level(s) and then try it. Be sure the new setting relate to actual monitor model being used for a better setup. Conflicts will arise if monitor is unable to lock-in and/or out of its range, thus new setting match. Continued operation can damage the monitor or flat out fail. I exclude any possible video card h/w issue. You didn't mention "why" you changed the monitors in the 1st place.

tada -----Willy Happy