Sunbaked

Hi I have a mercedes 300D 124 series that has done 400,000kms. It recently started running rough and I found that 1,2,3 injector were blowing from round the injector where it fits into the head. I have removed the injectors had them tested (OK) and cleaned the threads and the injector holes and refitted with new washers but they're still blowing. Any one know what might be the problem.