My sister has a 1995 Riviera Supercharged and it does the same thing. She never took it in to get it checked yet.
She has remote start and so it just keeps cranking and cranking if she doesn't listen and stop it. Like I say, she hasn't taken it in to get checked yet, and now that it'* been doing it for about 3 months when it does start it blows a cloud of blue smoke just as it fires up.
You might want get it checked out so this doesn'y happen to you. Looks like her valves or rings are going now.
If you find out what it is my sister would be more than happy to hear. If she gets hers checked out soon I will be sure to put the diagnosis on here.