DVD drive not recognised on 64 bit.

Hi all I have just rebuilt my pc with an AMD Athlon 64 bit cpu on an Abit AV8 mb (VIA K8T800 chip). Using ethier the 32 or 64 bit version of 9.3 the and the same DVD drives (DVD on hdc and DVDRW on hdd) as before there is a failure to recognise the first drive once the machine is booted up. If I run system repair on reboot it is fixed but the recognition does not usually survive a reboot. I have checked the caable connections and they are OK. Anybody any ideas as to the cause of the problem please ? Mike