LX160 Hydraulic Bog?

Hi all, My cousin has a 2006 Link Belt LX160 that recently started bogging down while digging or moving. The issue usually starts to show up after the machine has heated up. At first I thought it was fuel related (diesel injection tech for 20 years) changed the fuel filters, checked the overflow valve on the injection pump, fuel supply etc. After some googling I am convinced its a hydraulic issue. Like I said the machine, after it gets warm, will bog till it almost dies when you scoop a bucket of dirt or ground drive, then it will recover and blow a plume of black smoke and rev back up. The hydraulic pump was overhauled 3 months ago due to an external leak, but it has ran fine up until the last couple of weeks. I have read about an N9 control valve but I'm not sure on where the location or the testing procedure for this valve would be. Thanks, Jason