Well here goes.
I have just installed my daughters DVD drive and everything work perfect. Got to be a faulty drive. But no...........
I reinstalled the origanal drive so I could take the PC in for repair under warranty, and low and behold, that works perfectly as well now ????????
All I can think is
a: a loose cable connection, or
b: some dirt dislodged when th PC was tipped on its side to swap drives. or
c: the drive is faulty but until it plays up again I will leave it as it is.
thanks for all your help in solving this matter

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Stop press.
Ive just noticed the DVD drive is now running UltraDMA mode 4. Before the swap around I could not get the drive to run under anything but PIO mode. Maybe an OS issue after all. Swapping the drives and changing drivers may have fixed the problem.
Who knows ?????????????
Not me