The tachometer bounces after the engine is warmed up. Usually does it around 70mph. It will bounce around, and lurk steady about 500-600RPM's less than it should be reading while going down the highway, with a random twitch every now and then. Sometimes I see it bounce a bit in the lower gears when I get over 2300 rpm.

I thought it was the Crank Position Sensor, but the gap is correct, no affect on AC or charging.

Any ideas?

EDIT: This is a '97 12V 5-spd 2wd Dually with 3.55's

 

It should be your ecm/computer. I had the same problem, replaced the crank sensor which did not fix it. I was having a few idiot lights pop up as well.
A new ecm is approx. $500, but should stop the erratic bouncing tach.
Good luck.

 

The wiring to the sensor may also be the culprit. There are some places near the top of the head where they are known to wear through and cause problems. Might try the harness as well. The sensor may also be toast, gap isn't the only thing. Heat can cause the coils to separate in the sensor and it will short out.

 

Good to know. That bit of information might save you some money.