I have a 2005 pd140 automatic, when driving I'm getting a slight whistling noise and when watching the revs they seem to jump up asif the drive is slipping but it's only happening in even numbered gears, any help appreciated

When was the last time you changed the oil in the gearbox ?. If you haven't done this it would be a good place to start.

If revs are jumping in even gears then one of the clutches is slipping.

How often should the gearbox oil be changed? I though it would be a clutch slipping aswell, it seems to be worse when accelerating, any ideas of a price? Cheers for the help

dsg oil change every 40.000 miles (mines due soon ) around £200 stealer prices

Make sure that it includes the filter (it should if a dealer changes the oil).

Sounds like it needs a new clutch pack. The gearbox can easily manage 200,000kms in between oil changes, well mine did anyway. It just turned over 616,000kms today and the box is still sweet as a nut (knock on wood)

Any idea of how much a clutch pack is?

Not sure of a price for a clutch pack. My mates motor gave very harsh changes (also like it was slipping and grabbing) and was not that nice to drive until the gearbox oil was changed. Before throwing aload of money away getting the clutch pack renewed it might be worth a punt changing the oil and see if it makes any difference. BTW when I drained the oil out of the box it was a horrible brown colour and smelt burnt. You could of course drain a small amount off and have a look at it. 

Thanks for the advice, definitely worth trying to save a few quid, how easy is it to get to the drain point on the gearbox? I've never tried it before.

I suggest google. It will come up with several youtube clips showing you how to do it. As I say, drain off a small amount and have a look and smell at the fluid. Brown colour and smells of burning = not good, clear with an oily smell = ok.