Logically this problem should have been easy enough to sort but it has me beat right now.
An oldish cd-rom was not reading all cd's and was very slow when it did, so I changed it for a new one but no improvement. It was slave to the HDD.
I have since put it on a seperate cable on IDE 2, changed both cables and put it on both channels either as master or slave and none of these showed any improvements.
Have you any suggestions?

You did not mention a couple of things to check but you probably hve.Just in case.
Check bios settings PIO or DMA , new cables and newer cd rom should affect these. Make sure settings also match in Device manager.Does windows + Bios recognize the drive correctly?
Does the new drive have Firnware ? if so check for latest version.
If none of the above works dig deeper system problems.
Bios update itself .IDE controller ETC.
Good luck

I tried it in another machine and it worked OK so I presumed it was a good CD ROM. I tried a CD-Writer in this machine and it worked fine......which confused me a little,
I eventually got it to work when I disabled DMA in device manager for the cd-rom, although why the writer worked with DMA I am not sure.
Thanks for the your help.