that's got to be annoying. There's apparently a small screw that controls the tightness of the Dvd door/Flap, located near the flap itself. as for burning, it could be a few things. How much RAM do you have in your Mac Pro? Are there any updates to iDVD? what kind of blank DVD's are you using-some kinds do work better than others, so I've found-Verbatim and Taiyo Yuden. Also, try verifying and repairing permissions with Disk Utility. Click on your hard drive, then verify permissions, then repair permissions. You might try one of those cleaning dvd's with the brushes built in, or a can of compressed air (maybe the laser lens might be kinda dirty) . does your dvd drive read/write both dvd's and cd's okay? if yes, great, but if it's giving you problems (only wants to read/write either this or that, not both, or won't read commercially produced disks, then it may be time to get a new drive, or update your mac pro to Leopard/Snow leopard, which is $20. replacing the drive is easy, but I'd be inclined to see if it's still good. You can do this by going to the Apple symbol on the left top of your screen, then About this mac, then System report. Put a known good cd/dvd in the drive and see if it shows up.
hopefully some of what I've written helps you out/
John B