Float level and jetting are on the right track. Part throttle conditions are very sensitive to air fuel ratio and engine temperature as well. You might also try blocking the heat crossover between the heads and intake manifold.
Make sure your coolant mixture is not too rich as well. Too much antifreeze reduces the amount of heat that can be transferred from the cylinders. Many engines also require a relatively high temp thermostat to actually run cooler. The 195 degree t-stat slows the water down so that heat can be more efficiently dissapated in the radiator.
How can I determine this? Also, if it was too rich, what is the proper antifreeze mixture/ratio? And how do I determine what temp thermostat I have? I guess I have to pull it out?