I have a 1999 international with the t444e engine, 75,000 miles on truck. I a have bucket boom mounted on it to do construction work. I use the cruise control to ramp up the RPM's to about 1400 to operate my hydraulic boom. In the past two months my problem is getting worse to the point it is really loping or surging while I use the hydraulics. It will also surge slightly at idle without the pto engaged. This truck starts right up every single time and drives down the road perfect with plenty of power. A very slight surge can be noticed driving it at low rpm's. It does not seem to surge much at all when it is cold. Once it warms up a bit the surging begins at idle and then worse when I ramp up the rpms to run hydraulics. I have been everywhere on this thing checking and replacing things trying to fix this surging issue. New fuel filters, cleaned pre screen, checked wiring, new oil, replace ICP sensor, replace IPR regulator, replace fuel lift pump, replaced HPOP pump. I had it to 3 different, good truck shops. The international shop said that there is no more active codes after replacing the ICP and IPR. Next I started doing manual pressure checks on the HPOP. It seems low but not too bad. 450psi at idle, 1,100 winding her out to 2,300 rpm's. What should the pump put out for pressure. So seeming a bit low compared to another truck we tested to compare. I replaced the HPOP pump yesturday. No change. Pressures stayed the same. Seems as though I don't have as much pressure as I should at the HPOP. My next thought was o-rings on the injectors but it is just a stab in the dark. Seems to start and run good for bad injectors or o-rings not much smoke at all either. I hate to keep throwing money and parts at this thing just to try it but what else should I do. Hands up in the air, what should I do???????????