Twice in the last two years upon returning to Texas from Colorado I have experienced severe engine missing when I get back to a lower altitude. The problem generally shows up around Amarillo which has an altitude of about 4000 feet. The problem seems to get worse as the fuel tank gets closer to 1/4 full. I generally drive from Colorado to Amarillo in one day, stopping once for fuel. In my part of the country (altitude @ 1000 feet) I use 87 octane unleaded fuel and in Colorado I use 85 octane unleaded fuel. It was my understanding the engine fuel adjustment is made automatically at the higher altitudes so the use of 85 octane was OK. After I return home the problem clears up so when I take my vehicle in to the dealer they don't have any idea what the problem may be. I don't have any engine missing in Colorado at an altitude of @ 10,000 feet. Ideas? Maybe I should only burn 87 octane in Colorado?