1997 GMC Sierra Engine Miss

My engine has a miss. When at idle the engine has a miss to it. I have had the heads replace due to one being cracked inside where the valves seat is at on the number three cylinder and water was getting into the cylinder. The miss was a lot worse prior to the repair but after the repair it is still there. At one point after the repair it ran smooth but then the miss returned. After the miss returned, I replaced the cap and rotor, spark plug wires, and spark plugs. One thing I do see is a possible vacuum hose missing from the throttle body area. At times I do smell gas in the engine compartment but have not seen any signs of a gas leak in the area.

The check engine does come on but the check engine will turn off after driving the truck for a while. If I leave it parked for a while, say two or three days and turn it on to heat it up, the check engine light returns. At first it will blink for several minutes and then will remain steadily on. I have not had a chance to take it to get the code read, but was hoping of any possible ideas on what it could be. By the way, the exhaust has a certain smell to it which is hard to describe but is not like the usual exhaust.

Thank You