Kamping,
If you have the Cummins ISL that the brochure states you may have the same cause I had with my 2008 ISB. Cummins likes to use a BIG multiple connector to connect everything together. On my HR this connector (25 pin+, I think) was located by the right frame rail just in front of the transmission. My connector was not completely water tight and on extremely rainy days the connector would get wet and the torque converter would not lock up. I disconnected the connector, rinsed it good with brake cleaner and compressed air, wrapped it in heavy duty plastic and wrapped that with electrical tape. My problem was solved. I am guessing your connector may have some corrosion on the pins and this slight increase in resistance causes your intermittent problem or it could be in the connector at the ECU.
Randy
