What would cause a random misfire when driving at highway speed on 1999 Chevrolet S10

Runs strong in town and sitting idiol but when u get to highway speed it runs a little rough and throws a random misfire code and hills on the highway it does it more and looses a little speed. In town it is really strong and does not send the code hill or no hill I have changed plugs , wires it has all the power and runs Awsome until u get on the highway

by in Arnold, MO on May 01, 2013
1 answer
ANSWER by on May 02, 2013
Good scanner and a mechanic can run a 'movie' to view later for related problems. Also can check freeze frame data, just dont attempt to erase codes before test. Most likely ignition breakdown, more pronounced in higher gears, more load on ignition with lower rpm on highway.