thunderbolt imac 2405fpw

I just unboxed my new thunderbolt equipped i7 imac and now my external dell 2405fpw flashes DVI CANNOT DISPLAY THIS VIDEO MODE whenever the monitor is supposed to be asleep... it works fine otherwise... it's an irksome problem because the computer and monitor are in my bedroom, and flashing error messages do not make for a good night's sleep...

i do happen to be using a 3rd party connector to go from my mini video displayport (thunderbolt) to the monitor's DVI.. it was working completely fine on my mid-2010 imac.

i have this feeling somebody will just say, "oh, go get the $30 apple connector, and throw your $3 connector in the trash." but isn't there a better solution than that? and how can i be sure that will work properly?