As it’s a 164-480 I’m guessing it’s got a few miles under its bumper, I’d check the fuel lines, the rubber hoses go wonky on the inside, they’ll look fine from the outside, but they balloon up on the inside and cause fuel restriction, at 1400rpm on a hard pull you’re asking for a lot of fuel, so any fuel restriction will show up more in that situation, it’s a cheap easy fix and should really be part of SOP on an older lorry 
Either that or it could be any one of a multitude of sensors 