I recently been having problems with my rx8. I would turn on my car but it would kinda stall on coming on. I thought it was the battery so I had it tested and it was bad. I replaced it only this time to come to find out it was not the battery but the alternator too. had it replaced yesterday. Now i went to turn it on and it came on right but after a little bit it started to kinda want to turn off even though i got a new battery and a new alternator. As i continued to drive it would later turn off and i came to a light and was sitting at the light it would want to turn off on me so i had to keep giving it some gas to keep it on. Now it seems to want to turn off when I run it and im at a light or stop sign or something where im just waiting .

 

It could be a few factors causing this issue. And we need more info, such as year of car, transmission type, maintenance history and any modifications.

But when reseting the vehicle memory (unhooking the battery), the car must relearn the fuel trims. This can cause unstable idle. It usually takes 2-5 drive cycles to relearn.
I recommend you unplug the battery for 5 min (depress/pump the brake pedal to completely drain the memory), then rehook and go for a few drives (allow the car to warm up, fully cool down, then repeat 2-5 times). You SHOULD notice more stability.

If this fails, you'll need to troubleshoot to determine the issue.

 

Could have a vacuum leak, failing ignition coils, eccentric shaft sensor out of calibration, or a few other things. How old are the coils and spark plugs? They need replacement every 25k miles or so.

Any check engine codes?

 

Like soldier said, but the warm ups are not during the drive, they are before the drive.. and yeah, more data.

The stuff in the Congrats thread below may help as well.