1994 Pontiac Grand Prix 3100 V6?

I bought this 1994 Pontiac Grand Prix 3100 V6 with 110,000 miles for $1700 back in the summer of 2004 going into my senior year of high school. I needed a car that I could afford and that was reliable and this car was definitely a good choice. Since then I have graduated from high school and a year and a half VOTEC College and now I am out on my own at the age of 20. I was planning on dumping the car when I got my first job and buy something newer but I found out insurance rates would eat me alive at the age of 20 for a new car when I am not under my parents plan so I kept the car. Now the car has 153,000 miles on it and problems are starting to creep up on me. The only maintenance I have did on my car for 43,000 miles is change the alternator, tires, air filter, and change the oil and oil filter on a schedule of 3,000 miles. Before my problems I had no idea how a car motor worked but the last two nights I have sat down and researched on your website and on this one http://www.howstuffworks.com/engine.htm and now I fill like I have a hang of things. The problem: When I first start my car and drive it a little ways (less than a mile) and I come to a stop light and idle everything looks good RPMs stay at a steady 1000 but after I get the car up to normal driving temperature of 185 to 200 and have been driving it for awhile (30 to 40 min) and I come to a stop light the car wants to idle very very rough shaking my car. Sometimes I think it’s going to die but it manages to keep going. At this point the RPMs are bouncing around the 600 to 700 range and it fills like it’s a rotation where every so often it feels like its going to die but gets stronger then weak at a point and then stronger again. Another thing I have noticed is I need to put a quart of oil in for about every 500 miles I travel. I don’t have noticeable white smoke coming from my exhaust and I don’t have a noticeable oil puddle under my car where I park. Does the 94 Grand Prix have a common fault place that it’s know for leaking or burning oil? One more thing is I will be traveling on the highway traveling about 80mph with my cruse control set and every once in awhile I feel a loss in power like it might be a miss in one of the cylinders when this happens I usually freak and slow down to about 65 then gradually work my way back up to the 80 mph. And for the recorded my check engine light is on and has been on since I bought it. It’s not on when I first start my car but it comes on awhile after. I’m the type of guy that would rather spend more money and more time on something than bring it to a repair shop for less time and less money (which normally doesn’t happen anyway) I get a feel of accomplishment out of fixing something or learning something new. I bought these ( http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=110142028998&ru=http%3A%2F%2Fsearch.ebay.com%3A80%2Fsearch%2Fsearch.dll%3Ffrom%3DR40%26_trksid%3Dm37%26satitle%3D110142028998%26category0%3D%26fvi%3D1 )
service manuals off eBay to try and help me in my up coming project. They are the ones straight from Pontiac. I guess what I’m trying find out is the issues with the things I mentioned above and what should I all do in a tune-up after 43,000 miles. I’d rather spend a little money now (somewhere around $1,500 or less) than buy another used car that could have the same problems. I’ve seen people drive cars with 250,000 miles on them and I would like to be one of them. Thanks for your time I am very enthused to here a response from you. (Hopefully a quick one) I’ve been reading on your website for sometime now and you sound like you are full of excellent advice. Look for the generous donation coming your way. If you need more information to diagnose my car just let me know.