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this sounds like a lack of fuel and could be a lack of fuel pressure from the electrical pump at the tank.
If the connections to the pump 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 distributor (should be 5 to 5.8bar) 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 this is OK then remove the rubber intake boot from the fuel distributor and then unscrew on the top of the unit find the central nut , pipe feed into the fuel distributor and push down on the aluminum plate under the rubber boot you should see the main fuel metering pin / needle rise up smoothly as you push down the plate, if its sticky or dirty then its possible this is getting stuck.
If this is OK then remove all the injectors but leave them connected to their fuel lines and put all into a clear container then turn the ignition on and (having replaced the central feed pipe nut) lift up the aluminum plate all the injectors should spray at once and look for ones that aren't spraying as much as the others - and replace or ultrasonically clean as required.
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