Very common problem and not just on the 407. The 307's also suffered from this problem. I believe it's the heat affecting the actual liquid crystal unit rather than the electronics. I once tried fitting a substantial alluminium plate to the PCB in the MFD to try and take heat away from the PCB chips but it made no difference. All that was left was to buy a good replacement.
Be warned that you will need to use Diagbox/PP2000 to set them up for your car as they come shipped with all options disabled so none of your menu/stalk buttons will work to change language/time etc etc until a host of options are enabled.