So I started up my car this morning, and it idled around 800-1000, normal for the temperature in my area this morning. I backed out of my garage and stopped so I could close my garage and noticed that it was already down to 500-600 RPM, which was its normal operating temp.
I came up to my first stop,took it out of gear and the RPMs dropped all the way to 100 and was about to die. So I gave it gas to keep it from stalling.
I turned off my car then turned it back on, and it idled perfectly then I gave it a little gas and when the rpms came back down it dropped to about 300 then worked its way back to 500-600...so I continued on my way to school and noticed that once again if I didn't keep giving it gas it would die.

What could this be? This has never happened before. I'm thinking it's either a bad spark plug or a bad injector, mabye a bad air temp sensor? I really have no idea what it could be, I'm just guessing. Any help would be great.

Car: 1998 Civic DX Coupe - 5 Spd