I have used my Ender 3 for 3 years with the original 1.1.3 mainboard, driving it by USB from a Raspberry Pi running Astroprint. It has always worked very reliably but with noisy stepper motors. I have just upgraded to the silent running 4.2.7 mainboard and now it loses its USB connection randomly after a few minutes. The printer just freezes the extruder and bed temperatures are maintained but my Astroprint screen says it has lost USB connection with the printer. I can manually reconnect using the Astroprint screen and regain manual control of the printer to raise the head and return it to home, but the print is lost. Has anyone else had this problem? I am trying to determine if it is a firmware problem or have I got a faulty board? The new board prints reliably and silently from an SD card.