corolla 2.0 d4d help required please

[w13bby]

Good morning all. I hope this gets posted to the right section as i'm new to this.

I have a Corolla 2.0 d4d that has a running issue when warm. It's a 56 reg with only 71,000 miles with a full service history (80 per cent main dealer)

When cold it drives perfectly in every way. When given a good run so up to normal temperature it loses power up to 3,000 rpm then suddenly emits

black smoke and then pulls really well again until the revs go below 3,000 rpm. If you unplug the egr valve then all is perfect again even below 3,000 rpm

Ive already taken the car to a local well known diesel specialist and to RRG Stockport neither of whom could gets to the bottom of this issue. 

Parts changed: fuel filter + maf sensor (genuine one) + egr checked and apparently working fine.... everyone stumped....currently driving with egr unplugged

and drives perfectly but of course eml is on.