'91 420 SEL with 199,899 miles is sputtering and stalling

[fredsegal]

Hello again all,
Here is the scenario please let me know what you think;
While idling in either park or drive the car maintains a fairly steady 500-600 rpms but every once in a while the idle drops to almost 0 and the check engine light comes on. Sometimes the motor will stall but usually it will go to 1000 rpms before returning to 500-600. The problem is now occurring with more frequency and also happens when I decelerate or as I come to a stop.