2002 Buick Park Avenue Engine is hard to start when at or n

Engine is hard to start when at or near op. Temperature. The engine starts fine when cold. But, after it has been running and has come up to or near operating temperture, and it has set for more than 6 minutes, I have to try it 2 - 3 times before it will start.
Replaces the fuel filter, no help. Replaced the fuel pump relay, no help. Replaced the plugs and this helped for a while. It seems to getting worse.