Gas Recirculation Valve (egr Valve) problems of the 2004 Kia Motor Amanti

Dear sir: I am the owner of a 2004 kia Amanti. There are 43,542 miles on the odometer. The vehicle engine runs fine and then will suddenly go into lugging and misfiring at various speeds causing down shifting. When I turn the engine off and leave it to set overnight or for more than eight hours at work, the engine light remains on but the engine runs fine once started again. After a few more starts the engine light goes off. I took it to the dealership and told them of the symptoms. They ran tests and said they could find nothing wrong. They said bring it back when it does it again. Fine, so it goes into the misfiring on a weekend at night traveling the interstate. No chance to just leave the engine running for a dealership the next day. Now it runs fine again until another inconvenient situation. Problematically, the engine behavior creates a driving hazard at high and low speeds and could easily cause an accident. This is a serious safety issue that needs to be resolved by kia, not by its ignorant or ill trained dealer mechanics. Are you familiar with this problem? is it the computer module that may have a short or crack in the housing? misfiring is a timing issue and most surely goes to the firing order and timing controlled by the advance mechanism and the module. Or is it the throttle system? or egr valve or solenoids? do americans need to die before kia comes up with an answer? I need to get this fixed before my wife who panics may have a serious accident.