2004 impala hicups and sometimes stalls

My roommate has a 2004 impala and it had died on him so he got it towed to a repair shop, he already had a check engine light on for a long time and knew what it was, 02 sensor. so when he took it to the shop that is what they found and replaced it. drove it home and it died several times on him. He showed me and it would hiccup and the rpm’s would bounce around and then it died. My wife has a chevy and awhile back it was doing the same thing and it had a really bad spark plug so it was miss firing,so I checked his spark plugs and they looked pretty bad, 2 even had what looked like a spider web in between the gap.So I replaced them and I let it sit for 20 mins and it was fine. when i went to test drive it, it hiccuped while accelerating once, maybe twice. Let it sit and it did nothing, even while in park and bringing up the rpms. drove it around the corner again and it hiccuped again and then when I stopped the rpm’s bounced around like crazy and almost died. gave it some gas and then it seemed fine.

so that’s where I’m at, any help would be very much appreciated. and the only other thing he could tell me is that some technician said that his battery was not working very well when they did a test on it i guess.