Truck Repair: truck missing, cummins engine, tail shaft

Question
We have a 1997 Volvo with a m-11 cummins engine.  This truck has  a check engine light come on,misses some in the mornings but as the day goes on truck runs fine.  Codes show erractic speed but we have checked out all sensors and still cannot find a problem.  Any ideas what the problem could be?  Thank you

Answer
Are the codes telling you the "road speed"?  If so you need to check the pick-up wheel in the tail shaft of the trans where the sensor screws in.  If the Yoke on the trans is loose it will not hold enough pressure on the wheel for the speed sensor and give false readings. {basically the wheel will "slip" and show a slower actuall speed}  

The same code could come from a bad sensor/ wiring