Might be your burner. I have a Toshiba DVD burner and for awhile I used cheap DVD discs and they played on anything but got tired of having to clean the discs before I burned them. I use Imation now. Also remember not all DVD-rw's read on the first try sometimes it takes up to eight trys before the box will read them. Also the newer ISO's for xbox like to be burned with DVD decrypter.
Just an FYI... you're not actually using Imation discs... you're using whatever die Imation is currently buying. If you get anoter case of Imation discs, you might get a different die that will not work for you. You should buy dvd-r's based on die, not brand name, as very few (if any) name brands use only one die.
As for newer Xbox games needing or preferring dvd decrypter, where did you see that? I've never heard/read that before, and have made plenty of newer backups with no problems at all using Nero or RecordNow like I have been doing for quite some time now.
Crash - like Joymonkey said, be sure to finalize the disc. Also what are you burning at? I burn all my games/movies at 2x. In general, the slower you burn the less likely you are to have errors or compatibility problems. Give it a try burning at 1x or 2x and see if your PS2 will read it.