After many helpful suggestions and trying all of them i've still got all the problems (dozens of them, see previous posts) on my Mac (G5 imac Superdrive) So finally i'm going to (as advised on here) to buy an external firewire DVD drive and try a complete reinstall that way (superdrive is knackered)

Will this do the job?

LaCie d2 Dual DVD +/-RW Drive (Firewire) . I'm looking at this as if the reinstall works i'll be able to use it for backing up DVDs.

Many thanks again to all who've helped so far.

Kind Regards