I tried to put it into the drive and the engine shut off again?

The first time I noticed an issue was when the engine shut off after 8 stopped at a stop sign. I had to turn it off and crank it 3 times before it started and after that there wasn't an issue for a few days. Then a couple of days later I started the car, and it was working fine for a couple of minutes while I let it sit. I put it into reverse and then backed up. When I tried to put it into drive the engine shut off again. I only had to crank it once and it worked. I noticed it when it was in park for a little while the RPMs would fluctuate from below the.5 RPMs all the way through the 1 on the gauge. Today it didn't start until 4 cranks, it's a good bit colder today, it's in the 40's today but the other times it was in the 70's. I got to my university today after it eventually started, and it seems to be idling fine. I've been reading and from what I have read it seems like it could be anything from the spark plugs to the compressor to the fuel injectors. I don't know much about cars, so any help is greatly appreciated.