2001 Subaru Outback Battery?

About a month ago my car was chugging and then would stall out. Took it to the shop and they replaced some kind of plug. 1 week later the check engine light came on. But car ran fine. 2 weeks after that I went to start it car wouldn't start. I hooked it up to a battery and it started right away I let it run to charge it up. It did this for the next 2 days after letting car sit over night. Then out of no where the car was fine again no issues starting up. It's been 2 1/2 weeks and today the car didn't start so I jumped it with the battery again. This time my abs light, clock, electric milags gage and temp oil light flickered on and off and the rpms would go up and down. I got to where I needed to go but when I tried leaving again car needed jumped. I jumped it but the second I shut the battery off the car shut off. I had to let it run for about 20 min then just barely made it home. ( Was only right down road from my house). I haven't tried starting it since. What could it be? Battery? Alternator? Something more serious? I also have a video of the rpm and lights going nuts