UKPassats

I have a seat Leon 1.9 tdi alh engine, the problem is when I start it, it will run 100%, but after about 10 seconds it starts to idle very rough, shaking the whole engine and car, anyone any idea what would be wrong? As soon as I touch the trottle it revs smoothly.

Also the car had a replacement turbo just before I bought it.

All help appreciated.