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.