Hello, and thank you for taking an interest in my problem.
Thinking about the power supply, since I've changed everything (the RPI, the tuner, and I've obviously reinstalled Libreelec and Tvheadend. ) is a good idea. I hadn't thought about the power supply. So I changed it, and we'll see if that solves the problem. By the way, you've made me think of something else I haven't changed, and that's the Internet connection. It's very bad, I live deep in the countryside, at the end of the line. I have at best 400k/s (!), and with cuts. Can this have any consequences?
My RPI is a Pi 3 B, 4x 1.2 GHz, 1 GB RAM, WiFi, BT, there are no peripherals apart from the tuner, I work on it via the Tvheadend web interface, so not even a mouse or keyboard. It's just connected to the TV via Hdmi, and to the network via the RJ45 socket.
