Idvd Burning Question- Defaults to 8gb Dual Layer


I have just installed a G5 Powermac twin porcessor 2.5 Gbs ram at home, I love it.

Problem is burning an imovie - it indicates 'trouble burning disc' and hangs - it encodes menu, assets and audio quickly then hangs on the burn.

The 4.7 Gb discs can be burned subsequenly on my iMac.

When I return to Project menu it has defaulted to

8Gb dual Layer, what am I doing wrong.

Please help.