1993 Infiniti G20 near stall problem!

My 1993 Infiniti G20 only has 29,000 actual miles on the entire car. I bought the car a couple weeks ago from a towing company, the car had been sitting there for about 4 months not driven. I drove it on a 350 mile road trip to get it home. I didnt notice any problems, it drove great. Once I got it back to my town it started acting up. When I go to start my car, on occasion, it will turn right on, but the rpm will rev up to 1,200 rpms and then drop to 200 rpms and then rest at 1,500 rpms. When I put the car into reverse or drive, the rpms drop to about 700-900 rpms. The car will drive fine all over town, the only problem is when I stop or am idling at a stoplight or what not. The rpms will drop to around 400-600 rpms and the car will vibrate and the rpm meter will vibrate up and down. It feels like the car is going to stall but vibrates and kicks back in. I also noticed that my dashboard area vibrates when I accelerate, and then it dissapears once im up to speed. I dont know if this has anything to do with it. My car is low millage, but 13 years old. I have a brand new distributor, cv joints, tires, rotors and pads, and oil. Should I check my transmission fluid, or? Please help, it has me worried, thanks! :Cry: