Petri,
I've had issues with DVD+RW's, similar to what you state. They also appear to get much worse with time/use. Some players are also very particular with them. Try burning your project to a +/-R and see if you have the same issue.
I burn about 30 DVD's a week and my workflow is very similar to what pjc mentioned. PD7 to edit and create a DVD Folder and then ImgBurn or equivalent to burn DVD's from the DVD folder. I also use verify as I don't want discs returned and can't test them all myself. I've also found a rather large difference in number of coasters from various brands of DVD's. For me, Verbatim has been very reliable.
Jeff