I've got a BenQ 17" Flat panel which recently goes blank after more or less 5 mins everytime its used. I can use it perfectly fine for five mins or so until it just goes blank. The green light stays on indicating the signal is still there.
I had a Dell 2707 that started going bad after a week - initial symptoms were screen going blank for about 1 second, at random intervals of several hours. This gradually increased until it finally went blank for 5 seconds, then came back up with random lines and blotchy grey areas.
To make sure it was the monitor and not the video card I plugged in a spare monitor which showed a normal display screen, then plugged the cable back into the Dell which once again showed the lines and blotches. Then I plugged my laptop into the Dell - no change. Then I did an online chat with Dell tech support and within 5 minutes they agreed to replace the faulty unit. The new one works fine.
If you have a spare monitor and computer I suggest you try the above to make sure it is the BenQ and not some other problem..