Any ideas or experience with this issue?

I have a Ford s max 2.0 tdci zetec with 107k well maintained and generally well looked after. I have recently noticed a surging sensation when sitting at 70 mph plus. Although the speedo doesn't move it feels fairly significant (so much so my girlfriend has noticed it which in itself is incredible!). It accelerates well in 1st, 2nd, 3rd and starts to hesitate in 4th, it stutters and struggle but does continue to accelerate it then pulls fairly well in 5th and on into 6th.

The surging sensation is very annoying as it's at motorway speeds that it's evident. 

I have had it checked at the garage and no faults are stored, no split pipes and nothing evident. 

I bought the car with 85k on it and from the day I've owned it it makes a paaa chooo noise (awesome description) when you change gear. Sounds similar to the 190k boras with daft exhausts that infest my home town every sunday night.

Add to this the Abs/traction light is coming on periodically and the mpg has dropped from 42 to 33 and I'm starting to think it's all connected. 

My garage think it could be a clogged turbo and I'd like a 2nd opinion before I invest my hunnerds of pounds in replacing the turbo. 

Many thanks for reading and if you need any further information I'm more than happy to provide you with whatever I can.