about 2 months ago, i advance my timing to 19 degrees, and set my base idle at 800. Everything was fine, now since its cold, when i start my car up, It idles around 1500, then drops to 1000 in a minute or so. Then after about 10 minutes it drops back down to 800. However, when i put it in park, it idles at anywhere from 1600-2000. And for some reason my car wont idle at 1500 rpm exactly. Whenever it tries to idle at 1500, it keeps bouncing from 1200-1500 and sounds like it is going to stall. I always had hesistation at 1500 rpm as well, i changed the plugs to bosch platinums. What else could possible be the problem.

get some good ngk plats stock ones. get new wires cap and rotor. clean out the throttle body. make sure your timeing is at 19 for sure, get someone else to check it as well. but id guess your idle air control might be a little messed up /gummed up

read the tread I have up right now there some info there that might help u I still trying on mine
the tread reads "1500 idle

Maybe you forgot to plug the TPS back in?

no, the tps is plugged back in. I adjusted the timing about 3 months ago. It started doing this about 1 month ago