Cherokeesport1998

Just bought a 98 cherokee sport automatic 4 door. When i drive it runs great. I know that the exhaust manifold has a crack in it so its loud coming from the engine bay. I dont notice any misses or anything when driving. When i park it idles fine for a minute then i it goes into a rough low idle, it acts like its about to stall out then randomly revs up, then back to low idle then revs up again and so on. Like i said there is a hairline crack in the manifold but i dont think it would cause this. Also i have horrible fuel milage. Any ideas guys? Maybe a sensor or something?