I've never used the software, but this might help:
1. Is it possible that the video side of the menu system is only set for 15 secs? You may be using a still image, but even so, it has to convert that into a video format which means that you can probably specify the length that the picture will be shown. Extend the length of this, then you may be able to extend the length of your audio.
2. Possibly you are authoring it in NTSC format? Make 100% sure when you convert it that it is in PAL (assuming you are in the UK of course).
3. This could be for two reasons I can think of.
a. Your DVD is over 2hrs long and you are trying to fit it onto normal DVD-R. Perhaps try using a Dual Layer DVD. It is possible that the compression of the data is causing the artifacts.
b. The MPEG2 converter in Power Director is crap.
It may be worth trying the following though.
When you are exporting out your movie, instead of burning it to DVD, save it to your PC. The use a third party program (such as DVD-Shrink) to reduce it to fit onto a normal DVD-R.
Another method : Get the actual video edited first, then export as an AVI (Warning, will be a big file) then if you have Nero, use Nero Recode to make the DVD menu and author the DVD and see what happens.