Peugeot 407 engine/rpm surging

The engine revs fluctuate around 200 RPM at various car speeds whilst driving, for example if you are maintaining 30 MPH due to a speed restriction the car will lurch just like a learner driver is driving. This can happen at different speeds, not just 30 MPH.
I have taken the car to different garages and they have plugged in their diagnostic computers but no faults are shown and there are no warning lights shown on the dashboard.
Upon recommendation I have changed where I purchase my fuel, I have changed both the engine and gearbox oil and filter.
The air filter and fuel filter have also been changed.
The car had a new turbo fitted last September as it blew, I do not remember having these issues before the turbo was replaced.
The car is a 2.0 SW diesel type fuel, and the car is automatic.
One garage said it might be the glow plugs that were causing the problem but they were not sure and due to the possibility of shearing the glow plug on removal they advised not to proceed with the job as it could be very costly.
The car has only done 45,000 miles.