It is overheating and throttling. But 21.5w is a lot for standard 525mhz. What is the voltage you are running? It maybe set to high. 1150mV is standard. You can run with 1100mV on 525mhz I run my german bitaxes with 1100mV up to 600-625mhz. I heard the china bitaxes are bad quality compared to german or swiss made. But even with china built bitaxes 15j/th should be possible.