Cat 416d help and advice needed

I have owned a Cat 416-D for three years now. Sometimes it runs well and other times poorly. I have changed fuel filters a dozen times, cleaned the fuel tank vent, replaced the air filter, drained the tank looking for debris etc. I found a few old gas cap parts in the tank and a small tubular screen that looks like it should be over the fuel intake tube. The machine runs well at idle and it also runs well when hot (after 1/2 an hour). In between, it often stalls out or dies. During this time, I can put the pedal to the floor and it will not respond and will slowly die (sometimes), but at other times it will slowly ramp up in RPM. This behavior is on and off. I have drained some fuel out of the bottom of the fuel filter and found some little black specs that were attracted to a magnet (rust...?) but the inside of the tank looks very clean.

We are a long way from a Cat dealer and the one time I took it there, they couldn't find any problems. Can any of you please help me with tests procedures or ideas?

Kind regards,

JB

Cat 416d help and advice needed