Car starts then dies

My boyfriend has a 1999 Ford ranger. His truck will start then immediately sputter like it is about to die. When he lays on the gas to try and get more power it continues to sputter then eventually dies and his battery gauge keeps fluctuating.

He replaced the alternator a year and a half ago and the battery was replaced almost two years ago.

He started it yesterday and it died soon after. We jumped it and he layed on the gas. It managed to stay started a few minutes then died again and would no start. Any thoughts on what to check? He thinks it is a bad battery. I think it's something more.

1st step would be get the battery load tested. Most Auto parts store do it for free. If the computer doesn't see a consistent 12v, lots of symptoms.

if this was my vehicle the first thing I would eliminate would be the ignition switch, when the vehicle dies leave the key in the on position do not turn it off at any time or the test is no good, then pull the coil wire only one will be needed and test it for voltage, if there is no current your ignition switch is no good, if there is current while testing for voltage have someone giggle the key in the on position, hope this helps if nothing else it eliminates one possibility, good luck.