Help My 560 sel starts runs for a bit then stalls and is hard to restart
 

Hi, I have a 1990 560 sel 149000 miles
I dont drive every day maybe 10 times a month.
I start the car usually fires right up.
Runs for 30 or so seconds sometimes 2 minutes then dies.
At that point it is very hard to start back up.
I noticed today it feels like it is not running on all cylinders.
Then it felt ok.
What should I check in an order basis?
Also, when it dies it is like it is turned off (it just stops running).

Thanks Bryan