Thinking about it I witnessed a similar thing with a Gardener engined Leyland Titan. It had new lift pumps, new injectors (sprayers in Gardener manuals), injection pumps, even a new fuel tank. Turned out one of the drivers would dip the fuel tank with a rolled up new newspaper and bits of it would fall off into the tank occasionally blocking the stack pipe....the saga went on for weeks until someone cut open the fuel tanks and discovered them full of newspaper!