External DVD burner nor Internal CD burner working

Emac - running OSX 3.9 purchased October 2004
External LaCie DVD burner (USB and Firewire) purchased June 2007

Initially I had a problem trying to burn DVDs to the external LaCie DVD burner - the DVDs wouldn't burn giving me these error messages:

Buffer underrun error. Writing was halted because data could not be sent to the recorder fast enough to maintain the current write speed. Please try again with a slower write speed.

-50 interface error the connection is not stable

I’ve tried (unsucessfully):

Using different types of blank DVDs both RW and R
Writing at the slowest speed possible x1 and x2
Burning from Toast, Finder, Iphoto
I even tried re-installing Toast

I also tried attaching the DVD burner to a PC – it burnt the DVD but didn’t verify it sucessfully.

12GB of space are free on the Hard Disk so there should have been enough space to burn the DVD.

I then remembered that in the past the emac has once in a while had problems burning a CD but on a second attempt it’s been fine.

So I then tried to burn a CD using the emacs internal drive. I tried three times with no sucess and these messages:

The drive reported an error sense key medium error sense code OXOC write error

2nd time: Sector 623 is unreadable verification failed

3rd time (after burning a cd in Itunes) when I try to play it I get the message No volumes that the mac can read.

I am now worried that it is more serious than something to do with the CD or DVD burner.

Thanks in advance for any advice.