Try some carb heat. My O-320 behaves like yours, but I can use carb heat to "steer" the mixture.
At 7500 feet, WOT, I can get the cylinders to peak at just about the same time with the carb heat pulled on about 1/4. Play around and I will bet you will find a solution.
Good luck -send me a PM if you would like to discuss on the phone.
Amateur Plane - RV-9A N789PH - 2400+ Hrs
Amateur Radio - KD0CVN
Doggies Delivered - 25+
St. Paul, MN