I had an issue years ago with a LCD with blurred columns that affect text. Does the blury/smudgy text all blur vertically in line and repeat later on the screen with another column of blur/smudge. You may have multiples of these blurry columns.
The fix was to change the refresh rate in the video properties from the default of 60hz to 75hz even though its a LCD and not a CRT, this corrected it on that HP DC5000 computer. It seemed to be a refresh issue where there was an odd analog harmonic at 60hz refresh and 75hz setting made it go away.
Also some LCD's have Video Noise ( Phase and Clock ) adjustments when you press MENU and navigate to adjusting the settings and these can be used to sharpen and get rid of blur/smudge as well.
Also... as per ...
OK i did an experiment switching the monitors and there is no problem. Then I switch them back and the problem is back.
Question I have is... can these remain switched to solve this or? For the fact that you have one that seems to be degrading it may be getting ready to fail for good, but if you swapped the displays and its better, I;d remain swapped if same size displays.