2007 Mazda 6 accelerating 0-20 very slowly

Last weekend while driving 1 mile to the store, the car was accelerating slowly and speedometer kept jumping from 0 to 40 then back to 0 when I was probably going 20. I turned into a neighborhood, shut off the car, and it wouldn’t start back up. (I have a breathalyzer IID installed too).

I got it jump started and it started up but then turned off on its own 2 minutes later.

Battery is only 4 months old, went to swap it out with warranty, and the car worked completely fine for 2 days until the same problem happened, started up in drive way then died trying to back out.

Shop said it was the alternator. I got one elsewhere and had a local mechanic install it. Driving it home from his shop, it was accelerating slowly and it read that I was in 4th gear from the when it should show gear 1 then 2 then 4. Just says 4.

I don’t know if this is electrical or a transmission issue. I did have a transmission leak that I have to top off with fluid every week, but with that probably it was usually just rough shifting , not anything like this.