It would be really good if you took us back to the beginning. describe how your radio, the SDRplay and your PC are connected. Tell us about your antenna set up and the software packages you are using with the hardware. I have virtually the same set up except for an older model SDRplay (RSP1A). I am running win4yaesu, log4om, HDSDR, fldigi and WSJT-X. When I first started out with the RSP1A, every time I transmitted it would knock out my com ports. I added a couple of common mode chokes, and it solved the problem. But I am also wondering if the software you are using is triggering the MOX (or VOX) function somehow. It might be that your CAT setup is incorrect. There are a lot of things it could be in my mind, so more info is required.