What is the problem with erratic idle after a few minutes of run time in International 454 tractor Have drained fuel and replaced both fuel filters but the problem still exists Any advice?

My experience as a fuel injection pump specialist tells me that more than likely the injection pump needs to come off and be looked at. You could also have air getting into the system and one way to tell is to remove the fuel return line at the pump, start it up and look to see if the fuel coming out is clear or foamy. It's going to make a bit of a mess so maybe have a drain pan handy. If the engine is hard to start,that's another good indication of air. Usually with air in the fuel though, it won't run very good at any speed, not just idle. Hope this helps.