Our 04.5 Dodge with a Cummins did the same thing. It was the connection between the dash and the guage packet. Ours had a lot of dirt behind it. Once we removed the guage packet out, cleaned the connections and reinstaled, life was good.
No, not really. I had instructions from one of the diesel forums that I followed. It was a bit ago, so I don't remember all of it. My uncle likes to leave the windows down when the truck is in the field for some reason, so the inside of ours is dirty! So, it might or might not be your issue.
Being its intermitent, it could be the motor that drives the tach or somethng in the cluster itself.
My truck is a few years older but I had same problem with my cummins and it was the crankshaft sensor. the tach gets its signal from this and when they wear out the signal can ecome erratic, hope this helps just my two cents!