UK Car Repair Questions? Ask a Mechanic for Answers ASAP
I'm assuming that you haven't got any check engine light on so I'd begin by looking for a lack of fue lpressure from the electrical pump at the tank.If the connections to this are corroded or damaged then the pump could stop at any time or could not be running at full speed. Check that its relay switches in and out and the relay contacts are clean and bright – replace the relay if in any doubt and do the same for the fuse,Ideally measure the pressure at the inlet to the main engine fuel pump and if this is low check the pump as described and also consider changing the fuel filter as if this partially blocked this too will reduce pressure.
If both of these are OK then also check the connections to the EGR valve, if either the vacuum pipe of electrical connection are damaged / corroded then the EGR can be on all time which certainly will hurt performance. Its also possible that its a fault with the EGR valve so its worth removing the valve and cleaning it out with brakecleaner, if it looks particularly clogged then replace it.