When started the RPM goes back and forth between 1,000 and 3,000 rpm when in park. When put into gear the transmission slips when rpm is high and engages when RPM drops. I can drive the car but its fun to keep it going forward.
First thing I'd do is check the transmission fluid level and have the vehicle scanned for any possible codes and check the transmission fluids quality and color etc too.
Hope that helps and best of luck.By the way a basic tune up and check of the air filter , vac lines etc is a good idea since its idling like that.