Problem with 1990 Grand Prix

I have a 1990 Pontiac Grand Prix 3.1L V6 164K miles and there is a problem with it idling normally. I recently baught it and got my license suspended so I havent been able to drive it for about 3 months and previous to that it sat for almost 2 years. The oil has been regulary change and all the fluids are up to par but at the same time it doesnt idle right and due to that the car dies. It will start and idle normally for about 3 minutes and then after that it will jump rpms. It will go as low as 800 RPMs and jump as high as 2500 RPMs and after jumping so high and dropping to so low it just dies. It started to do it about 3 months ago after I purchased it when I was driving, and during the time of these jumping RPMs it wouldnt allow me to accelerate and it would die. I have changed 1 of the 3 coils, I havent checked the injectors but I have replaced 3 of the 6 spark plugs, but after the spark plug change, all that did was eliminate the miss it had. I have had a computer run on it and everything the computer read was being read normal. If you could please assist me in telling me what could potentially be wrong it would be greatly appreciated. Im somewhat mechanically inclined and have a very limited amount of tools. Thanks