I chased the same problem for days. Tried a new CTS, fuel filter, fuel pump, coil, etc. It acts more like a fuel problem than an ignition problem.


It is the Ignition Control Module under the distributor cap. When the dielectric grease dries out (older trucks), it turns to powder and stops insulating the module from the base of the distributor. When the motor heats up, the chip inside fails. Seems fine when it is cold but fails when warmed up.


All the major parts stores can test it. The test heats the part too.

 

A 6.5 is a diesel. No distributor and this thread is 3 years old.