91 Jimmy won't start when warm

I have a 91 Jimmy with a vortec 4.3 L and will start perfectly fine if left when left for a few hours. But once I've been driving it for awhile and turn it off and try to start it again, it will fire up for a a couple seconds and then just keep turning over. I'll wait about a hour or so (once it's cooled down) and it will start up right away. I really don't know what this is caused by and was wondering if anyone out there has ever had the same problem and how it can be fixed?