Did you check the compression with the injectors out? That is the first thing I would check with an idle only problem.
The reason that compression is more critical at idle is because compression is what you got after all the air is squeezed into the space at the top. If you have leaks this pressure is reduced. If you have leaks time is the next factor. At higher rpm the same size leak has less time to affect things.
The same concept follows for pump element check valves inside the pump. If the column of fuel leaks back into the pump between strokes different fuel quantities are delivered. Since this is a leak and time makes it worse it affects idle more than at higher speeds.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
[This message has been edited by stevebfl (edited 03-30-2000).]