I had the same exact issue. My intake manifold wasn't properly torqued. Make sure you go through your manifold bolts and make sure they're tight. If it still happens, it might be time for a new gasket.
2000 TJ Sahara, Spartan reman 4.0, 3 speed auto, Bestop Trektop NX, C&C Fabs 3" flare tube fenders, Rampage euro grille guard, stock suspension, 31" Duratracs.