Car starts easily. Runs at 1800 RPMs for several minutes. Then it abruptly..

What is your question?

drops to around 1100 rpms, runs rough for a second or two, then back up to 1800 rpms and runs smoothly. It keeps going through this pattern until finally, it abruptly stops running. It will start up easily everytime. Then back through the same pattern. Thanks for your help.