Rough idle

I have a 1990 Mazda b2600i 2wd and I got her the other day and I turned her on and it idles high and it starts to idle high it goes from 850 rpm to 3000 rpm without me touching the throttle and then when I give it throttle it drops and then raises again and when on idle it sounds like its misfiring but when running at 3000 rpm it runs like a champ what can cause this I'm thinking fuel remember truck was sittin for like 6 months before it came to my hands

Check the intake manifold gasket.. they have a tendency to cause a vacuum leak after sitting for a while.. just spray the intake where it meets the head with carb clean and see if the idle changes..if it does you need a gasket.. if not, could be a temperature problem..they high idle when cold too..if you have a coolant temp sensor problem it'll high idle too.. but if it sat than check the coolant level to make sure its full then try the intake just to weed it out..could be a few other things but the intake is a good start