Dead Disco

I really need some help with my 2003 TD5 Discovery. About 3-4 weeks ago I noticed starting off from a standing start there was no power at all until the turbo kicked in. I though it may have been the MAF so I disconnected it. The difference was even more noticeable so I thought it mustn't be that. When I reconnected it everything seemed to be back to normal. Good power starting off ( traction lights coming on when leaving the junction ). Then about 3 miles later it was back to its gutless ways again. Now it's even worse today. It's automatic and once it reaches lock-up at around 55mph that's it. If you hit a hill it drops a gear and revs it's head off. At present it's doing about 15 miles to the gallon. It's well serviced ( oil and filters every 5000 miles ) and has 120000 on the clock. Anyone any ideas what could be wrong?

Did u check for oil in the ECM's red plug? ...anyway, to read fault codes would help u much