Problem playing DVDs on 17"Al

[Selwyn Ward]

I have a Philips DVD recorder that writes DVD-RW.  The discs played
perfectly in my Ti500 but when I put them in my 17"Al, the display only
shows the top half of the picture with the bottom half blacked out.  Anyone
any idea what's going on?  It's not a problem unique to my PB as I found
exactly the same problem putting the discs in a colleague's 17"Al.