Please remember that any mention of competitors, hinting at competitors or offering to provide details of competitors will result in an account suspension. The full rules can be found under the 'Terms and Rules' link in the bottom right corner of your screen. Just don't mention competitors in any way, shape or form and you'll be OK.
My Dell ST2420 does this from time to time, Is yours permanently like that?
I to have multiple monitors and only does it on one of them, I would send yours back, after you are sure its not a driver issue or graphics card fault.