Cobra 200gtl High Power Transmit problem??

Question

This radio connected to a 25 amp power supply and running high power on Cobra 200gtl.. When transmitting,on high power, the needle moves properly and output power is good for a few seconds then within 2 seconds the power drops off rapidly , needle in meter then does slowly drop away under ssb modulation till it does not react at all. On low power the transmitter is ok ....Any solutions??