Pontiac Woe's...

I recently bought a 91 Pontiac Grand Am (2.5 automatic). The man that I bought the car off of said that it hadn't ran in about a year, so naturally I figured it would need to be ran to blow it out and get the old gas out of the system. It ran okay, but missed a little and just not quite right. I figured it was because it had been sitting for a year. Well since then, i've ran it a lot and ive already filled the gas tank up once, and it hasn't gotten any better. It still misses, and runs a little rough. The car runs great under acceleration, but when idling it runs like crap. Any ideas? Another problem is that the speedometer doesn't work. It stays on 0 until I get to around 40-50 then it lifts off the peg a little... I naturally figured it was the speed sensor, since thats usually the problem of a nonfunctioning speedometer. Put the new one in today, and it didn't fix the problem. It's still doing the same thing. Please help! Thanks, Gary