Mac Mini with Cinema Display

Have just bought a new M1 Mac Mini, and it's working well apart from an intermittent display problem. The Mini is connected to an Apple 27in Cinema Display via its USB-C port and a USB-C to Mini DP adaptor. The native resolution of the display is 2560 x 1440, and 95% of the time, this resolution is achieved. On the other 5% of the time, the display reverts to low definition 1280 x 720 - and looks terrible. When in this low def mode, the option to reset it to its default is not available in System Preferences > Display, so the only way I've found to restore the 2560 x 1440 setting is to restart the computer. Furthermore, the display exhibits intermittent red flashes in Zoom sessions which may or may not be part of the same issue. In all other cases, the display works perfectly and is completely stable. Does anyone have any suggestions on a fix?