GMC Van

I asked this question a few days ago but did not hear a peep from you. Without a donation, I guess it takes a few weeks, eh? I have a 1998 GMC 3500 van with 140,000 miles on it. It starts ok the first try, but it seems when it gets warm it is sluggish in starting. I replaced the plugs, the fuel filter, and had an injection cleaning, but it still does it. What could be the problem?

Steve