Rough idle only at start up

Just bought a 1999 Ranger XLT 4WD 5 speed with a 4.0 L engine. It runs rough at start up then clears up in a few seconds and runs fine. This is not in cold temps but is happening in 70 to 80 degree weather. What could cause this?

kinda had same prob. I changed the fuel pressure regulator.

Thanks for your reply, maybe that will help my truck as well.