SDi low speed problems

with a cold engine the car is fine but as it warms up the whole car seems to shake when using very light throttle from between 1300 and 1800rpm.

it happens in every gear execpt 1st and goes away if i slightly dip the clutch or if i floor the accelerator.

trying to describe the effects is hard, the car seems to lurch or surge with quite some force.

the car pulls fine after 1800rpm with no problems at all, the car starts fine either hot or cold.

there are no fault codes present.

the fuel filter has been changed but has not solved the problem.

any sugestions please as its driving me crazy.