If you check and reset the injection pump drip timing. Yet engine still runs badly you may wish to check the milli volt output of each glow plug looking for any major discrepency. Swap glow plugs and injectors into the strange reading cylinder if one is indicated. .
If problem remains and compression across the engine is simular. Someone has probably tweaked the element on the pump or that specific element is defective in some way. Perhaps even just wear from use. If nothing else the careful testing will indicate the pump at fault or clear it. Also the increased pop pressures might be at the edge of what the pump is willing to easily supply on perhaps one element.
There is a slight chance if the problem is broken down to one cylinder you might be able to correct it yourself. There would be nothing really lost as otherwise the pump is on the way out to a shop for a check. If it proves to be the fault by test.