New to this, truck running rough
 

Im new to this Forum. I am having problems with my 1988 Ford Bronco II, it has a 2.9L v6 engine. It runs fine when I start it up, but once it warms up after 5 minutes or so, it starts to run rough and idles rough. Nomral idle for it is just above 1000 rpm, but running rough it stays under 1000 to 500 and just goes back and forth. After a minute or so its back to normal running conditions. When driving it will do the same thing ever so often and it will have no power to it what so ever. I am thinking it may be a vac leak, but im not to sure or where to start looking for the problem. Thank you.