Blown Head Gasket or Cracked Block

Check your rotor, cap, wires and plugs. It doesn't take much for something to be amiss (pun intended) here. You also might want to pull the distributor and take a real long hard look at the lobes. One might be worn. Still not running right -- you might do a compression check. As long as you had the valve cover off did you check valve lash?? You might have a sunken exhaust valve from running unleaded.