I would try to swap in a known good idm. Correct me if I'm wrong but I believe the buzz test is performed through a different part of the Idm then when it's commanded to "run".The icp pressure would bleed off it the Idm where commanding the injectors on while cranking.
After replacing my lpop. Had trouble with same thing long cranks. I took lpop apart and filled with grease and problem solved. Two years later and starts on first crank. How long has truck ran since lpop.