DONT be sure that you are not losing OP unless you have verified it against a manual guage. You can buy one from miller or just a pod mounted guage as I did. I was (and still am) having the same issues you are having but mine occurs at start-up. I installed the guage and sure enough, the idiot light wasnt fooling me; there was no OP.
The sending units are known to become faulty at times... but never assume this as it is the HEART of your Cummins!
Cerb, I think mine needs an exorcism! First, i'll swap the OP (easiest) then, if that doesnt work... drop the pan

not looking forward to that!