I am SOOOOO sick of reading forum after forum after forum after forum saying change your media, xxxx is a bad make of disc, delete your plist file, reboot it, reinstall the OS, boot from external and repair permissions on the internal, lower the recording speed, etc etc etc etc etc etc. It's great that users are trying to help other users, but where the **** is APPLE in this process? NONE of these suggestions worked here and Apple are clutching at straws and throwing out red herrings to keep people busy on the rare occasion that they actually ring and ask for help.
I have had this problem for months, it began JUST after my warranty expired on my 27" iMac i5. I was burning discs with Finder daily for a year, perfectly. I rang support today and was told, sure we can log a problem report and try to help fix this, send us £35 please. I can't afford that, I spent all my darn money on discs thanks to a million suggestions about different brands on variuos forums like this one. Verbatim are the best, no no, the real best are only made in Japan and you have to get them direct from the factory in Osaka, blah blah blah!
Some say its a faulty superdrive. So how come my external brand new LG burner i got the other day is doing the exact same thing. Weirdly, around the same time, iDVD changed. It used to burn in around 3-4 hours, in hundreds of cases, never longer than 5 hours. Suddenly, it says it needs 38 hours to burn a 1GB AVI file. Thats ridiculous, so clearly this is not hardware, its software.
I bought THREE new burning programs, every one of them run for hours, getting my hopes up, only to fail at the burning stage with either drive.
I have tried every brand of discs you can buy without a £2000 plane ticket to Japan. 😉
One thing I haven't tried is strangling someone on the alleged "Genius Bar". They may as well be at the "bar" for all the good they are with this problem!
If anyone has any REAL ideas of what might be causing this, I am all ears 
Yours,
Ex-PC user who loved Mac to bits for several years, but is soon to be ex-Mac user, not that anyone seems to care like they used to. That was always the risk of Apple getting more popular, instead of being niche, different, ultra caring towards their customers, they just become like everyone else in the mass production market, bag the money and run. Sad, very sad, almost as sad as their stock price lately.