at times the voltmeter will read 12v, at times 15v, but it dies unexpectedly

What is your question?

So today while idling in my driveway my Express van died immediately started right back up but it had been reading below 12 volts on the voltmeter. The battery is good just had it tested and it's only 6 months old. The alternator is literally a week old and I have the 145 amp or volt whatever it is and I recently replaced the fuel pump which was not the problem because it was the fuel pump relay that was bad it's replaced as well

How long has your 2011 Chevrolet Express 2500 had this problem?

1 week