Have an older 310d deere backhoe that runs almost perfect at 2000 rpm, slow it down a  little and it acts like it runs out of fuel and dies.  Then to get it to start I have to unscrew top of filter to relieve pressure and it starts up and runs again.  Motor runs smooth when it runs, has fresh fuel, changed filters twice, drilled out fuel cap so I know its not creating a vacuum in tank, checked fuel supply for leaks-    Will run all day at 2000 rpm with no load,  will run for 5 minute intervals at 1200 rpm then need to release pressure to get it to start.   could it be the fuel pump or maybe the injector pump?  need some guidence.