A secondary ignition misfire can cause the conditions you are having. The ignition system components are under very heavy load when traveling 40-45 mph in overdrive. Shifting down to 3rd lessens the load on the ignition system and the misfire goes away. Chances are pretty good the a spark plug or spark plug wire is at fault. Only a complete inspection of the ignition coils, spark plugs, and spark plug wires will confirm this. If "carbon tracking" is found on any of the spark plugs the spark plug wire to that plug should be changed also. If your spark plug wires are original in may not be a bad idea to replace all 8 plug wires if fault is found with any of the wires.