I think on the age of this vehicle and the possible additional problems you may have [crank damage,weak oil pump] i strongly suggest a second hand engine lots of breakers will have one 150 200 quid and pop a new cambelt and clutch for loads more motoring :D
I look after hundreds of tdi engines most die because of broken cambelts and waterpump failure get them changed along with ALL the pulleys and tensioners and youll see 200k :wink:
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try cleaning out the egr valve with carb cleaner inside,if symptoms persist try unplugging the airflow meter multiplug the drive and see if power returns. 4 injector defects at once is tommy rot...