Possibly the new monitor is less capable than the old one and needs to work at lower frequncies.
I am not familiar with CentOS but as far as xorg.conf goes I suggest you edit it and reduce the higher end of the frequency ranges:
Identifier "Generic Monitor"
DisplaySize 280 210
HorizSync 30.0 - 70.0
VertRefresh 50.0 - 100.0
Try 30-60 for HorizSync and 50 to 60 for Vert Refresh. (or look at the manual if you have it)