hesitation and erratic idle when vehicle is warm

I have a 99 Ford Windstar that we purchased a month ago. It has run perfectly for the month but the other day after driving about 10 minutes at 45, it felt like the van was missing a bit while driving and then I felt a hesitation when I took off from a stop light. This continued for the next 15 minutes or so and then stopped. Within a day or two after this it started idling erratically when stopped at a red light. Again, only after the vehicle warms up and if I put it in park or neutral the idle is perfect. If the van is cold it run perfect. I only feel the hesitation at the same time as the erratic idling and only when accelerating.