1997 Grand am

my daughter has a 1997 grand am with a 3100 v6.the tach was spiking around and the car would die out.She was driving yesterday and the car died completely.I went, jumped it and was able to get home.When i checked the battery with a fluke meter while running it had 12 volts,I changed alternator and now has over 14 when running.She went to leave and after driving for 10 min, she called to say the car was again dying out?When she brought it back it would hardly run and was sputtering like it was having a timing issue.I left it out over night and put it in the garage today and the car started right up like nothing.I replaced the mass air flow sensor and took it for a ride and when it got to normal temp it died out and the tach was moving around? i started it back up drove home and let it idle for 15 min?It was fine?No check engine light is on,smooth idle,no miss?

Welcome to AF!!

There are two possibilities here. First, did the security light come on when it died? If it did reset the system and see what happens. The other and the probable cause is a bad crank position sensor. The sensor will overheat and cause the car to stall. It usually won't set a code until it completely fails. The key is the fact that once the engine cools it restarts and runs well until it gets up to normal temp.

If this happens only when engine gets warm I would replace pcv valve, (PCV can act fine when cold but once it warms up it can start sticking and start creating vacuum leak, and rpms will jump up and down since ecm is trying to command iac to open and close to compensate for the vacuum leak PCV valve $2.39 at A.Zone