2000 Pontiac Sunfire Stalls While Accelerating?

So I am a young woman who knows virtually nothing about cars so be gentle please! Ha. Anyway, my 2000 Pontiac Sunfire will randomly not accelerate while I am driving. I was initially rolling up to a red light so took my foot of the gas at about 20 mph, then the light turned green so I tried to hit the gas but nothing happened for about 5-7 seconds. So I was just coasting, but then I hit the gas and everything was fine. It did this a second time on the way home. Two days later I went to start my car and it wouldn’t start. Nothing was happening–no noises, nothing. So I got my battery tested–Autozone tested it and said it was bad, but after replacing the battery nothing happened still. Then a “mobile mechanic” came and stuck a screw driver into the starter to short circuit it so it made a sound like it was trying to start, but didn’t. Then after a few minutes he looked and my radio was on and it started with the key. Since then it has not started only one time when I used a screw driver to start it myself, but when I was on a steep incline last night it kept not accelerating if I made it work too hard. I would be at ~65 mph and it would just quit so I’d pull off the side of the road and it would drop to ~40 mph for ~5-7 seconds, then I’d hit the gas again and it would go just fine. It did this about 3-4 times. One time all the lights on the dashboard flashed on when it wouldn’t go. One time it just completely stalled on me when I was driving. I pulled off the side of the road and started it again and it was fine. Any thoughts???

I forgot to mention that it has 118,000 miles on it and I got the fuel pump replaced about 3 years ago.