I am not quite sure I understand what you're asking, but nevertheless the chip is populated on the PCB with its PowerPAD soldered (if this is what you mean). Do you think this is not enough and thus normal operation is affected by the heat? To tell you the truth, the possibility of overheating is something I haven't thought of so far. And in fact, I don't know of any way to improve the power dissipation capability...
One more -maybe stupid- question : Is it possible that the switching currents flowing through ground plain can cause the voltage level of ground to rise slightly above zero and thus disabling the power save mode of the chip? The max input low voltage for the PS/SYNC pin is 0.4V. Of course, this case is not observed when measuring with the voltmeter during operation, but I don't know what else to think of.