30" Cinema Displa y is blank

My wife has a 30" Cinema display. Recently it would often stay blank upon booting up - no error message, no logo, nothing. We looked around various forums and the consensus was the power brick, which we replaced and thought everything was OK until the other day when it started happening again. The computer works when we switch to a regular, non-Cinema monitor (but we have to do a power cycle; i.e., we can't simply switch monitors after getting the blank screen on the 30"). Since it's not consistent, we're not sure if it's the videocard or the monitor, but I'm suspecting the latter. We haven't (yet) had the problem with the replacement monitor. Has anyone else had a similar issue? It's an expensive monitor so I'd like to fix this. Thanks for any help