It sounds like the engine is tightening up a little when it's hot. Have somebody load test the battery to 4.2 volts and see how much current it's drawing from the battery. If the current draw is over about 300 amps, the problem is with the engine, not the starter. Also, try pumping the gas pedal a few times before a hot start to create a momentary flooding condition. If it cranks faster while clearing the flood, your ignition timing is advanced too far. If it's got a stomp start, try checking cranking speed with the ignition off.
Jerry
