i sory my friend but it looks like you either got air or most likly water in your fuel system, or you are running out of diesel. i would go get some fresh diesel in the other tank ( make sure its good maybe a different station than normal) and then go run it for awhile so that the pump and injectors get some good fuel. if that doesnt do it than i would start looking for leaks. most likly problem is the filter is letting air in or the water separator is leaking or worse clogged.
my idle is staying steady the tach is just moving! it runs execelent! it stays at a steady idle and goes above 1500 rpm just the tach is messed up or the sensor is messed up. there is nothing wrong with the way its runs.
change tach sensors
ill try cleaning it of in the morning. it has a small bare spot on the insulation. i put some silicone on it dont know if that was a good or bad thing to do lol.