idle drop question
 

I have a 88 Cherokee Laredo 4x4 that a bought a month ago. I have a question about something iv noticed. When I first start the car it will idle and stay on, but when I press the gas the RPMs drop to low hundreds and the car sounds like its shutting off, but it never does. It always raises the rpms and goes, it never dies or anything. It's weirdnd i don't know its a problem or not, maybe water pump? Idk ohh and it only does it when the car is first turned on, it never does it after its running for a min or two
Thanks guys!