Volvo Repair: 96 850 int hard start, negative battery cable, crank sensor

Question
Regis       
 I have an intermitent problem. I run a repair shop
  and im having a difficult time finding this problem.The local volvo dealer and a few other techs
haven't been able to solve.
  To start;  1996 volvo 850 non-turbo,auto 123k
 This is newer style non egr engine.used motor was
 installed by another garage and sent to me w/ this
 problem.the harness and flywheel of the used motor
 were installed w/ motor.within every 10-15 starts
 motor cranks no start. diagnosis reveals no spark and no ground signal from ecm to grey wire
on injector. after several cranks engine will
start and run fine for hours.it may take 10-15
starts to do this again. no check engine lite
no codes stored in ecm. i have cleaned all grounds
and replaced negative battery cable.from another
96 850 in good running condition(customers)
I have swapped crank pickup,cam pos. sensor,ecm
w/ same part#,coil unit,grey "suppresor relay"
and 1 injector.I also inspected the flywheel for
damage as best I could without pulling tranny
no apparent damage.I even pulled apart ecm box
checked for broken damaged wires or terminals
then ran a ground from neg battery terminal to
black and brown ground terminals on ecm.
I am running out of ideas (and patience)any help
would be greatly appreciated  

Answer
THEY MAY HAVE DAMAGED THE HARNESS SOMEWHERE.WHEN IT CRANKS THE TACH SHOULD COME UP FROM ZERO TO ABOUT 2-300 THIS MEANS THERE IS A CRANKSHAFT SPEED SENSOR SIGNAL TO THE ECM.I WOULD BE SURE THE GROUND BY THE IGNITION COIL IS TIGHT.YOU WOULD NEED TO CHECK FOR THE SIGNAL FROM THE ECM TO THE COIL WHEN IT DOES NOT START.I HAVE NOT SEEN A PROBLEM SUCH AS THIS MOST COMMON IS CAM OR CRANK SENSOR.i WOILL LOOK INTO THIS MORE.