My truck will start but goes dead and dies shortly after

I started my truck one day after work was going around the building
went to dim my lights the radio went black and then came back on. I
parked turned it off went in and clocked out and when I came out it
wouldn't start. So we jumped it off which took longer than it should it
started I drove to the end of the parking lot. Used my turning signal
the radio went black again and made it less than a mile my head
lights and tail lights went off and it died.  Came back the next day
within less than 3 minutes it jumped off and drove fine for three days.
Drove to work when I went to leave it started fine but about two
miles down the road the radio went black again and lost my head
lights and tail lights and it died again after I let off the gas enough to
make into the gas station. Between times it done this  a relative of
mine took the neutral cable off the battery while it was running to
supposedly test the alternator. Myself I know very little about
vehicles. Any ideas? My cluster it out on it it's an 04 Chevy silverado
4.3 v6 with 140k miles. I'm gonna attempt to have a code check run
tomorrow. I hope someone here can help