Cylinder not firing.

I have a Olds 98 with 170,000 miles on it. The car start and runs good with no warning lights or that type of thing. But twice now when I have gone to start it the car will turn over fine but will not start. Both times this has happen at night. When I have gone out the next morning to see what is wrong with the car it starts and run fine. It has not start twice now with about three weeks in between. What should I be looking for that could be causing this problem?

How do I check these when the cars is running fine?

How do I check these out when the car is running fine?

Engine Mechanical problem
1989 Oldsmobile 98 6 cyl Front Wheel Drive Automatic 35000 miles

I just bought a 1989 olds98 at an estate auction. The car has 35000 miles on it. It will start if I spray starting fluid in it but then dies once the fluid runs out. I have not worked on this car yet in fact I haven't brought it home yet because it won't stay running