7" HDMI LCD (C) Waveshare Problem

Hi there,

I have a Pi 3 with a Hifiberry Amp2 and want to connect my 7" HDMI LCD © touchscreen.
I have added the following lines to the config.txt
according to the Waveshare guide

max_usb_current=1
hdmi_group=2
hdmi_mode=1
hdmi_mode=87
hmdi_cvt 1024 600 60 6 0 0 0
hdmi_drive=1

After reboot the screen works completely fine but when I reboot it another time it has the same problem as before that it just shows me stripes. I have to do the same thing all over again.

Does anyone know what the problem might be?