It amazes me that people seem to have an obsession with the EGR valve and think it's the cause of all sorts of problems. Its purpose is to reduce Oxides of Nitrogen (NOx), which is the harmful content of the exhaust. It does not serve any other purpose. The valve should only be open when the engine is under relatively light load, ie when cruising, so that its effect is not noticable by the driver. It should be closed when the engine is idling and at full load. If it was open at these times then the engine would be starved of oxygenated air, and this would cause a loss of power. Othere than that it should not cause any problems. If the valve moves freely through its entire range, and is clean then there really isn't anything else that can be wrong with it. I think garages like to blame it because they don't have any other answer.

A loss of power on a diesel engine will essentially boil down to a restricted air flow, loss of fuel delivery or a blocked exhaust. All of these should be checked and eliminated first.

Like I and Railroad stated eliminate probables.

Fuel starvation would be my first check.

And as this is unlikely to generate any fault codes it's worth checking for blockages in the lines or air gettig into the same.