1991 TD122 Intermitten throttle response, cutting out etc...

Hello, I have a 1991 White/GMC with a Volvo TD122. It's a farm truck, yesterday it started cutting in & out like it was starving for fuel or air at 1/4 to 1./2 throttle but it will run fine a full throttle, it's worse in 1st 2nd or 3rd driving around in a lot but not as bad out on the road in 4 5 6. It's also worse after the engine gets good & warm. It idles fine, it did cough a couple times when coming to a stop. I blew out the air filter & changed the fuel filters but that didn't help anything. Both tanks are over half full of fuel. I checked intake, turbo air tubes & didn't hear or feel any leaks. Any thoughts or ideas about things to check would be appreciated. Thanks

 

Would anyone know what fuel supply pressure it should it have out of the pump & the correct boost pressure? Thanks