zabriskie

hey everyone, I have recently bought a 1994 subaru legacy 5 speed wagon. I've been driving it for about a month but have encountered some problems with the idle. After warming it up, it seems that the normal idle is at about 500 or less rpms. Then after driving a few miles, stopping and taking it out of gear, the rpms go all the way down and it seems to stall, however it does not and the rpms go up high then back down to about 500-350. Also, it seems the a/c and defrost are affecting the idle as well. What could this be from? SHould I have a higher normal idle to begin with? I've tried putting stp fuel injection cleaner in the tank to no avail. I hope someone can give me some advice. thanx