1992 Pontiac Transport idling

I was having idling problems with my 1992 Pontiac Transport GT (3.8 engine). It was running fine up until a few weeks ago and then it started stalling when I would take my foot off the gas. I thought it might be the fuel pump or filter, so I changed the fuel/oil filter/pump and it still stalls. I left it idle in my yard for over 25 minutes after the changes and it didn't stall. I drove a few miles down the road and the van stalled. I would like to know what else I could try to fix the problem.
Thank you, Carroll