Ok so since I changed my LIM gasket and replaced the spark plugs a misfire occurred on the first start up. Then it stopped and went away until i drove it 900 miles to Montana. Then it misfired and had a code PO300, which is multiple misfire. Once I got to where I was going I was driving around town and on my way to the part store to clear the code it went away. And only when I drive at a constant 80MPH for atleast 50 miles the code comes back. But then after driving in the city for a while it goes out again. They are new spark plugs and wires. Anyone know what is wrong? Thanks