This can be verified one way or another by reading the millivolts from each glow plug. You have to have a standard refference plug or move one plug from cylinder to cylinder as your constant calibration refference.
If the glow plugs are not very old you could do a quick scan and only investigate further if the cylinders indicated quite different voltages. The resultant voltages indicate the running temp of each cylinder. Or simply put it's general contribution to the overall engine. (power balance)
This system will also indicate a poor injector as well. Just swap injectors to determine possible pump or injector fault. Thats if the resultant initial voltages indicate a problem of course. Just a system to help localise or eliminate a problem at minumin cost. .
The gentleman I quoted that pulled his pump and took it in. He would have noticed the much lower temperatures (voltage) from the cylinders getting inadaquate fuel to burn at idle if he had done this test. Post any results if you decide to do this test. Also if you need any clarification do not hesitate to mention it.
Could of course be many other things like uneven compression. The milli volt test should also indicate any suspect cylinder or cylinders to follow up on with a compression check. It can be deadly accurate if applied properly with a little thought.
You have done the valve clearance check I see. This is always important in a case like yours before anything else. .