Kia - Rio :: CEL Codes P0300 And P0456 - Difficult To Start After Filling Up

2006 Kia Rio LX with 78K miles,

I recently performed several repairs in the car including replacement of the timing belt, all other drive belts (power steering, air conditioning, alternator).

Recently, the car has developed a new problem where it intermittently has been difficult to start, but only after filling up the gas tank. The engine cranks over okay but it seems like it might have vapor lock. It seems to occur every time we fill it up. Rarely, the car has appeared to run rough for a few seconds, the electronic control module seems to make an adjustment and then it runs better. The gas mileage has decreased from typical 28-32 miles a gallon down to 23-25 miles per gallon, despite getting new tires for the car.

Additionally, the check engine light has come on intermittently…once reset it stays off for up to two weeks then will come back intermittently with the following codes:

P0456 which indicates a problem with the fuel evaporation system, which may be related to the gas cap (sometimes when this comes on, the gas cap appears to be loose, but other times it appears to be tight).

P0300 which indicates in engine misfireP0301 which indicates in engine misfire in cylinder #1P0302 which indicates in engine misfire in cylinder #2The first time that I received this warning, I replaced the 4 spark plugs and the check engine light did not come back on for a couple weeks.

I have also previously received a code for faulty oxygen sensor temperature regulation for the forward most oxygen sensor. From what I have read, this may be related to oxygen sensors were related to an exhaust or intake leak. I have attempted to carefully inspect the intake and exhaust systems and I don't find evidence of leak. (From what I have read, a lean fuel condition which may be due to a vacuum leak in the intake manifold or unmetered air getting past the airflow sensor, or an EGR valve that is stuck open are possible causes.

I have visually inspected the 4 coils/plug wire units and they all appear to be intact and since the engine does not run rough (it idles smoothly, seems to have normal power etc.), I believe that they're all functioning.Any thoughts about what should be my next step to diagnose and to fix this problem?Replace gas cap?Replace oxygen sensors?EGR valve?Could this be related to the timing belt replacement (CEL did come on prior to the repair but gas mileage and problem starting after fill-up has started after the change)? Carbon canister or solenoid valve for evap emissions needs replacement?(I have never been in the habit of "topping off the tank").