ur 425 will all of a sudden drop app 100 rpm then speed up . Sometimes engine fault will come up but not always.The next day engine fault may come up regularly and engine will not surge much.Cummins engine. Even has technicians stumped.
turn key on hold diag button down till new menu pops up.
Scroll down to engine and hit prog for a second the first code should come up write the info down.
Hit up button and write down all the rest of the codes however many is there can store up to ten.
Post what you find here and I will give you the meaning of each code.