1993 Dodge Dakota stalling problem

Hey folks, got a 1993 dodge Dakota 5.2l v8 automatic. The problem I'm having is as follows: the truck starts and runs perfect, when the engine is cold.(although there is a pretty loud hissing noise like something is leaking)As soon as it warms up the hissing noise goes away and the engine will have stalling problems. If you give it a little gas in park reverse neutral and drive, then let up it will die. It doesn't seem to die if the tranny is in 2nd gear and up and I'm going 40km/h and up. the rpm needle will go down to about 400 rpm then bounce back up to regular idle 800 or whatever it is. If the truck dies it would start right back up to normal idle. Also in first gear say in a parking lot if I gave it some gas and kept my foot on the gas it won't did. If I take my foot off the gas and give it a pump of the throttle as it was dying it would return to normal idle and wouldn't die. Lately though the pump of the throttle won't keep it from dying and it's starting to die in 2nd and third gear when I let up off the throttle. It rarly idles at normal rpm when it's warms now and dies all the time but again, it runs perfect when it's cold. I'm thinking there might be an intake manifold leak but who knows. Anyone have any thoughts?