Disco 3 TDV6 Struggles

Hi, We've just acquired a 2009 Disco 3. it's giving us issues when driving, the rpm dial starts struggling between 1500 and 2000 rpm. and then the suspension light comes on. Mech diagnosed it and said it was the injection pump which he replaced along with the diesel filters. it behaved for a little while but now the problem is back. Any ideas on what it could be?

Whilst it's struggling does it emit black smoke?