2002 Dodge Ram 1500 high then low idling issue?

I have a 2002 Dodge Ram 5.2L V8. Over the past 6 months my truck has had days where it will be at idle then the idle (rpm's) kick up then down low, up and down, especially frequent when just starting up but tends to happen anytime, just not everytime. I have heard the issue of a vacuum leak, whether that's the answer or not, but have no insight on how to fix the issue myself. Is this the problem? Also, I have had instances where I will be coasting down a steep grade road and place the truck in neutral and it will still out on me. Is this related? Any idea and steps to self-correct the issue is greatly appreciated.