While driving my tachometer keeps dropping to zero.

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

you have a bad low idel sencer

Have the battery tested but feel you may have found the problem.

This could be caused by a bunch of thing's , fuel system , engine sensors , ignition system , PCM etc.... Testing is the way to eliminate guessing . Viewing scan data is the best way to diagnose these type problems .
047 Engine Hard to Start or No Start Scan Data for Ignition
048 Engine Hard to Start or No Start Scan Data for Starting Problems
002 Fuel System Tests Start Here

Your best bet take it to a qualified repair shop .

Most likely the fuel pump is worn out electricially or
binding & stopping mechanically

Symptoms point to the cluster microprocessor. How was it proven? See diagram below.

Okay, jack the front of the car up and apply ebrake, make sure front wheels are off the ground have some one inside the car start the car and place in drive and if the axle spins then it the axle check both sides you will see the inner joint spin or the shaft going to outter joint spin but not wheel, also with the car on the ground and in park if you can push the car while in park then its an axle that broke but if it doesnt move and you feel it locking up in park then it may be the trany,

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?

TRY GETTING A NEW BATTERY AND LEAVE IT UNHOOKED FOR A FEW MINUTES THEN UNLOCK DRIVER DOOR WITH KEY AND IT WILL KEEP THE SECURITY FROM ACTING UP

Have your transmission fluid and filter changed. That should correct the problem.