Problems burning a DVD


Hello all,
I switched over to Mac from Windows just a couple months ago when I got my Powerbook G4, so I am Mac newbie.

I am having trouble burning DVDs. I have a Mashita DVD-R UJ-845E burner and will use iDVD or Toast 6.07, but neither works for me.

When burning iDVD it usually says "errors were found in the burn process" and gives me a multiplexer error, and in Toast a Medium error. I am using Memorex DVD-Rs. I don't know if the DVDs aren't the right kind or I am doing something else wrong. I can't even burn some pictures on a DVD. I have been successful burning CDs, but not DVDs.

I really would appreciate some suggestions/advice.
Thanks a lot.