The Tach flutters, the Truck shutters and sputters, then the tach drops to zero and the engine runs fine, then the tach flutters again, the engine struggles the Tach runs normal and engine runs fine. back and forth when driving on Freeway primarily.

Bad ground wire somewhere in the dash .

Have the automatic idle speed control valve cleaned.

What year is the vehicle ? The tach gauge probably is going out . GM has had insturment cluster gauge problem's . Faulty stepper motors .

sounds like cam sensor bad

yuck had elec problems take for ever to run through harness but that is what you have to do wire by wire testing for drops and if it doesnt happen when testing you will have to do it againg n again but in process clean and check connectors also

I have had the almost same problem twice. A 'thump', like the engine missed a fire and tach drops. Seems OK after restart. Electrical? Timing chain slip?
Has 135,000 mile. Any clues?

wire that goes to the starter not the main wire there is a smaller wire that sends electric from the switch to the starter solinoid might be shorting out (skinned wire between the starter and the ignition switch/fusebox

must be losing ground on it check body ground

1.Start by changing plugs and wires. 2.Check for vacuum leaks. With the engine running, wigglr every hose you can see, to make sure nothing is loose or cracked. 3.Change fuel filter. 4.Check timing. 5.Check air control valve. 6.Check idle control sensor.

There are 2 things that cause the tach to do this, one is the tach itself has a problem or a connection to it, the other is a leaky plug wire, it sends out radio noise, the tach picks it up and the needle spikes up and down, saw it more than once, sold a customer a tach and it was a bad plug wire.