Well you don't say what engine you have fitted, but we would suspect that you have a DPF (diesel particulate filter) fitted to the car. If so then it sounds as though it is getting blocked. By driving the car at high revs can sometimes clear the system (like a chimney); but you should carry out a 'regeneration'. To do this you need to run the engine at high revs (between 2000 and 2500) for approx 30 mins. You can do this yourself by taking the car on the motorway.

See www.dpfcleaningsolutions.com/clean-your-dpf.