In the OP there was mention of stock hard fuel lines being replaced with rubber lines. I have never heard of that being an acceptable replacement on an IDI diesel, unless you are referring to the return lines from the injectors. It would seem possible that if the stock hard lines from the IP to injectors were replaced with rubber lines that the pressure from IP would expand the rubber lines before causing the injectors to pop. That may be causing less than adequate fuel delivery to combustion prechamber for throttle position.