I have a 2001 GMC Sierra 2500HD.  When I got her back from the dealer after a maintenance Ck - she hasn't run right.  The last year and a half - I will be driving and the engine shuts down - completely!  The engine cranks but doesn't start UNTIL I remove the IGN relay, reinsert then she starts.

My friend and teacher worked for GM and Chrysler for over 35 years and taught big-block engine classes.  We have:

1.  Replaced all the plugs
2.  Replaced the fuel filter
3.  Replaced the screws to the battery with the correct ones.
4.  Changed and inspected all the fluids.
5.  Replaced the IGN and Starter Relay.
6.  The Alternator and starter work fine, as well as the battery has the volts.

When she does shut down, at attempting to restart, the engine cranks - But, I put a portable jumper and she starts immediately.  Duane, and all the other "guys" are scratchin' their heads.  I have to say I am not a mechanic - I'm learning - but this has the men stumped.  Any suggestions?  This is starting to get dangerous.  

Being a novice, I may not have used the correct terminology in describing my truck's symptomotology.  Is anyone up for teaching? I'd really like to solve this problem.  

You may have already checked, but my 2001 2500HD started to stall / hard start, but it would run fine most of the time. After the dealer (under warranty) tried several things, the truck finally picked up a trouble code for a bad crank position sensor and that replacement solved the problem.