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.
I have a 17 inch Dell CRT monitor (not flat). It's a few years old. It's a little bit of a long story but I am having problems with my new computer so had to hook up my old one which has serious start up problems. While restarting it several times to get it to start windows my monitor started to freak out. The green light started to flash and it started to make a constant clicking noise. I turned it off, unplugged it, etc. Right now it's not hooked up, I am on another older monitor. But when I just tried to plug it in again to see if it stopped it is still clicking. Any thoughts?