Assune you have a 300TD not a 300D (suggest you update your profile )- it makes a difference in trouble shooting suggestions. Not an expert but - have you checked out the EGR system? It's possible thet the EGR 'plate' has almost closed, and if so that would be a good culprit for what you are seeing.

If it does turn out to be EGR related then you may want to consider an 'off road mod' that puts the EGR plate into a full open position.