433MHz XD-FST receiver modules oscillate when powered w/o RF input signal

433MHz XD-FST Rx modules oscillate (after about 2-3 seconds after +5V power application) when powered w/o RF input signal. Otherwise (when both Tx and Rx are powered), they work fine. In my application, I need the Rx module to be always powered (to receive) and the Tx module powered when I want to transmit. Can anyone help me, please? THANK YOU IN ADVANCE. Lee Webb, Colorado USA 303-438-8142 LeeWebb314159@comcast.net