Stutter returning to idle from boost!?

Got a 300tdi disco 1.

When I've been testing it as I've been rebuilding it bit by bit I've noticed that if its rev'd up to 1800rpm it returns to idle perfectly normally. But anything over that (where boost starts to kick in) when I release it will drop to idle but then stutter for a second before levelling back out. It doesn't stall or anything. Just hear it get a little hicup and the revs drop from 850 to 650 for a second.

Any ideas before I go to deep into madnes trying to find the cause. lol. I figured it must be vaccum related and boost compensator issue perhaps?

Just a thought could it be the diaphragm in the fuel pump has got hard.

Mmm that's a point! Maybe I'll replace it just to be safe thanks