Im working on a model fd40zt-8 with what appears to be a fuel issue. The unit is running in cold weather between 0 and 15 degrees fh lately. The unit will run anywhere from 4-6 hours and then loose power to the point of barley moving. Change out the fuel filter and it will run like a top with full power for another 4-6 hours and have the same problem. There are multiple machines running off the same supply fuel and no other issues. This has continued through a few tanks of fuel now. I removed the fuel level assembly and looked down in the tank. Its clean. Dipped out a sample and it appears to be fine. Its had 7 different fuel filter changes now with 3 different brands of filters being used. When it happened I had the customer loosen and remove the fuel cap and then reinstall it in order to rule out any vacuum being created in the tank and not venting properly. I had the customer loosen the fuel filter the last time it happened and then screw it back up tight again without changing it. When they did this it ran good with full power for about an hour and then had the same issue again. When you remove the fuel filter during the problem time it is completely full of fuel. I removed the line from the filter to the injection pump and blew the line out. I did notice during that time that the banjo fitting on the fuel filter has a washer/gasket to seal it and that there was a tiny portion of the gasket missing, but it seals fine. This is a cummings diesel motor. Has anyone ever run into something similiar or have any suggestions other than what I have done?