Do you have the option of using another input device CD/DVD player? You could use the CD player and run a DD ProLogic Signal throught he receiver and see if the center channel drops, you could also just go buy a cheapo DVD player that has optical out which I'm assuming your using, or either way coax/optical, and see if experience the same problem.
If the center channel continues to drop, well then it's probably a receiver issue, if not well problem solved DVD player is the culprit. Just keep receipt and take the cheapo DVD player back, head down to you local authorized Denon dealer and pick up the new 2910 !! great upscaling DVD player with SACD/DVD-A capability at a great price !!!!
P.S. have you tried other outputs, i.e. you have the same problem using optical and coax outputs on the DVD player.....?