I cannot speak from experience, but several on here have recommended testing the glow plugs. They say that they should each be 1 ohm, or 2 ohm as a maximum.

I had air leaks upstream of the pump and it would barely get to 2k RPM and ran ultra rough. Swapping to a clear fuel line upstream of the pump or using a clear fuel filter can test for this, and finding the leaks is usually easy.

good luck,