2002 Ford Ranger 02 ranger idle when starting warm engine.

my engine is warm and has been running for a while. i turn the car off and then get back in and turn it on a few mins later. it starts, but the rpms are very low and sometimes the engine even stalls out. if i give it a little gas. the rpms jump up to about 1500 where they usually are when you first start a cold engine and evrything else runs normal after that. it only happens when u start the engine and its already warm. it has been happening more recently now but not all the time. i have changed the plugs at 75,000 miles. checked for vaccuum leaks and even tried fuel injectore cleaner. it has not happend but 2 times this month. other than that the truck runs perfect. no problems. if anyone could help me i would really appreciate it.thanks.