Renault Scenic 1.9 RT DCI 2000 ?

For a few weeks I was having a power loss now and again. Then turbo blew so replaced it. Now there is no guts in the engine until the turbo kicks in at about 3,000 revs. The management light also comes on when giving a bit of power. Had a diagnostic check which stated glow plugs, so changed them, but problem still the same. New diagnostic states no problem. Any ideas?