Flickering LCD when using > 5v power supply on a Pro Mini
If the LCD has a backlight you are absolutely over heating the regulator on that board by running it at 9 or 12V: At 12V with 100mA for the backlight and using 175C/W thermal resistance for the regulator:
Tj = 25C + ((12V - 5V) x 100mA x 175C/W) = 147C
That regulator goes into thermal limit at 125C.