Sporadic sluggish engine start

About a year ago I could not start my Buick Century 2002. It was towed to the dealer, who said that he could not find the problem because it started right up.

There was no problem until last week when I turned the key and the engine started so sluggishly that it stalled after 2 seconds.

I kept trying and eventually it started normally and I was on my way. Since then, no problem.

Are the 2 incidents related? Any idea what’s going on?

What should I have checked out?