My engine seems like it maybe miss firing or not getting enough fuel at times,or may cut off at the red light or at stop sign while sitting idle 3.8 v6 series 2 monte carlo

How long since a tune-up? Any applicable trouble codes? Check and clean anything to do with idle air system. Remove the iac and clean it. Clean the throttle body and any small passages.
Use a gage and check proper fuel pressure. Tbi--9--13 psi. If spider fuel system around 55--60 psi.
You can check idle rpms. Sometimes I raise the idle, a little, in drive, just so it won't stall everytime you stop at a red light or stop sign. The factory idle specs for a high mileage engine, may not work very good?

serial number/keygen for registry reviver

The battery was a red herring - it would not have anything to do with the problem.
Faulty spark plugs could cause a miss but probably not make the engine stall.
What you described is a classic example of a faulty idle air control valve or a dirty throttle body. The computer controls idle speed by regulating how much air enters the intake. It opens the control valve to raise idle speed when necessary.
I would check the control valve and the throttle body.

simple test for a vacuum brake booster
star and run the engine at idle without applying the foot brake
if it gets worse when the foot brake is applied , it is a vacuum leak in the booster ( replace the booster )
otherwise check for failed vacuum hoses including under dash control lines , vacuum leak at the manifold or EGR unit

try this,reset you onboard computer by disconnecting your positive cable from your battery for 10minutes. if you've replaced any sensors at all,your computer will not accept them into the[system] until you do this.if this does not help resolve the issue,then it may only be a bad idle control sensor. does the idle go crazy when you stop at stop signs or red lights? does it[the idle go up and down when in park?] if it does,then replace the idle control sensor on top of engine.[silver,or black sensor with only one pug in]autozone,acvance auto parts can point it out for you] cost of the part$20.00 or less.2bolts and 1 plug in to remove. the disconnecting of your battery pos.side will make all the check engine lights go off,but if you have an issue,they will come back on.thank you for choosing fixya.com

if it sits there and seems to rev up and down, it is probably your throttle position sensor. Mine acted up very bad, expecially when leaving the highway back into city driving when at a stop sign.

Do you check for codes every time you have a problem?
You know it isn't temperature related.
You know that pressing the gas pedal won't change computer input under certain conditions
How do you know you have fuel pressure? Do you drive with a gauge taped to the windshield?
Why didn't the shop repair it?
I would get a wiring diagram and check all grounds
Then scan the Throttle Position Sensor. As in, put a scanner or oscilloscope or volt meter on it.
Go through the entire system until you resolve it

You could have a bad or intermittent fuel pump. Not a cheap thing to have.

An OBD 2 scanner can read fuel pressure on the drive to see if your fuel pressure drops off at odd times to confirm. It can also tell if there are transcient vacuum leaks at idle which the engines idle air control cant compensate for.

Do the plugs, wires and cap too.

Ignition system. For failures that cut out like this is a common sign of a ignition system failure

Change the fuel filter. An put some injecter cleaner in it. With some good gas.Then hit thr highway an blow it out.Not to fast. lol